Abstract Assimilation model is a theoretical model concerning the change process of psychotherapy, which put forward by William B. Stiles and colleagues. The model suggested that, in successful psychotherapy, clients followed a regular developmental sequence of resolving the problematic experiences that they brought into treatment. According to the assimilation model, the change process of psychotherapy undergoes eight predictable stages. The presence of the eight stages and their developmental process has been empirically supported by a series of studies which conducted by foreign researchers. To verify the model, a case study of a girl, who gained success of change after 8 sessions, was conducted with assimilation analysis. The client’s name is Li Sha (a pseudonym), she presented with sleeping problems and a lack of motivation at work. After eight sessions, she gained a successful change. Her sleeping problem was alleviated, she could allow herself slow down, and she learned to take care of her needs. In her own words that she has learned to balance the needs of herself and others, do what she could do. Three investigators read and reread transcripts of different sessions of the therapy, tracked 2 major themes, one theme was called “must strong and be perfect — slow down and keep peace” and the other was “must take care of the others, others important than me — take care of my needs”. According to the themes, three investigators excerpted 68 relevant passages, rated each passage according to the Assimilation of Problematic Experiences Scale (APES) and the Markers of Assimilation Manual. To increase the rigor of the qualitative case study methodology, we chose three independent raters who were not familiar with assimilation model rated the passages (taken out of context and randomly ordered). The consensus ratings of the three independent raters were highly correlated with the investigators’ consensus ratings (0.90 and 0.85), so the reliability of the investigators’ ratings was high. The ratings of assimilation tended to increase across the session process (the correlation coefficient were 0.89 and 0.91, respectively), which demonstrated the APES’s construct validity. For the limitation of the length of the article, only the first theme’s passages were presented to illustrate the assimilation process of Li Sha’s problematic voice named “slow down and keep peace” by her dominant voice named “must strong and be perfect”, yielding a more complex and flexible self community. These passages demonstrated the eight predictable stages of resolving the problematic experiences. Consistent with the assimilation model’s conception of successful psychotherapy, the APES ratings for each of Li Sha’s two themes tended to increase across her 8 sessions. The high correlation with the independent raters’ consensus ratings suggested that the investigators’ consensus ratings were not unduly influenced by their theoretical expectations. These results verified and modified the assimilation model again, and lead to more capacity to generalize the model. The results strengthen the efficacy of the model which claiming to be pan-theoretical and to apply to successful psychotherapy in general.