There is a collective agreement about the necessity of content knowledge (CK) as a prerequisite for pedagogical content knowledge (PCK). However, most PCK models lack explanatory power because of the missing link between CK and PCK. Thus, the study firstly attempts to develop a theoretical model that consists of knowledge of students' learning difficulty (KSLD) as a plausible pathway between CK and PCK with the conceptual change research entry. Secondly, it tests the causative relationship between CK and KSLD of PCK. The CK for this study constitutes acid rain, as well as the concepts of photosynthesis and cellular respiration. Seventy-three preservice science teachers (PSTs) participated in this study. The study used open-ended surveys and vignette-based individual interviews to measure PSTs' CK and KSLD, respectively. The results indicate that (1) PSTs' CK and KSLD are low. (2) There is a significant correlation (p < 0.01) between PSTs' CK and KSLD. (3) Compared with a low or moderate level, the high level of PSTs' CK is a significant predictor (p < 0.01) of their KSLD. Qualitative evidence supports preliminary quantitative results of the study. The study provides a PCK model that indicates KSLD as a plausible pathway to connect CK and PCK, using a conceptual change inquiry as a point of entry. The study implies the proposed PCK model based on conceptual change learning is useful for future teachers in responding to student learning difficulties. Besides, the PCK model is a fertile ground for robust research.