Prospective memory is to remember an intention at an appropriate moment in the future. Prospective memory tasks can be more or less important. Important intentions often get more attentional resources in order to be successfully remembered. However, sometimes, it is not possible to neglect the ongoing task even if the prospective memory task is important. In prospective memory research, common manipulation methods include relative importance instructions (i.e., emphasizing the prospective memory task more important relative to the ongoing task) or absolute importance instructions (i.e., emphasizing the prospective memory task itself). In most of studies, relative importance resulted in enhanced prospective memory performance but came at costs in the ongoing task. However, it is not clear to the influence of absolute importance effects on ongoing task performance. The multiprocess view suggests that depending on task resources demands, people rely to different degrees on monitoring versus spontaneous retrieval for prospective remembering. So far, only two studies (Einstein et al., 2005; Walter & Meier, 2015) investigated the impact of absolute importance and cognitive resources on prospective memory performance,and had inconsistent results on ongoing task performance. However, the absolute importance instruction uesd in the first study is similar to relative importance instruction, and the manipulation method of cognitive resources used in the second study may be ineffective. The aim of the present study was to examine?the?interaction between absolute importance of prospective memory and cognitive resources occupied by ongoing task.?In this study, absolute importance was varied by instructing participants that remembering to perform the prospective memory task was very important in half of the trials (high importance condition), whereas not important in other half (low importance condition). In addition, we adopted the Stroop task as ongoing task to manipulate cognitive resources occupancy rate, which contains congruent level (high resource occupancy condition) and incongruent level (low resource occupancy condition). The experiment conformed to a 2(absolute importance: high vs. low) by 2(ongoing task congruency: congruent vs. incongruent) within-subjects design. The prospective memory task was to press a specific key if the participant happened to see one of four target color(i.e., pink) during performing the Stroop task. Results showed that the prospective memory performance in the congruent condition was faster than the incongruent condition. For the accuracy and RTs of the ongoing task, the interaction between absolute importance and ongoing task congruency was significant. The subsequent analysis revealed that in the congruent condition, the ongoing task performance with high importance of prospective memory task was better than that in the low importance condition. Under the condition of not congruent, the ongoing task performance with high importance of prospective memory task was poorer than that in the low importance condition. The present study suggests that ongoing task congruency affects prospective memory performance. Importantly, cognitive resources occupied by ongoing task moderates the effects of absolute importance on ongoing task. In the congruent condition, increased importance of prospective memory task may enhance the performance of ongoing task; while in the incongruent condition, increased task importance may interfere the performance of ongoing task.