Accurate assessment of the severity of psoriasis is important in daily practice and clinical studies. However, the assessment of psoriasis area and severity index (PASI) reflects the physician's experience, and thus evaluations by physicians are inherently subjective, with intra-rater and inter-rater variability. To elucidate the effectiveness of PASI educational lectures and the use of reference photographs on the improvement of accuracy and reliability in PASI assessments and to develop effective educational programs for PASI assessments. We performed a before-and-after comparison study during nation-wide PASI educational workshops. The participants were asked to assess the severity components of PASI (erythema, thickness, scale, and affected area) three times: in the test administered before an educational lecture, the test immediately after the lecture, and lastly the test with the use of reference photographs. The improvement of accuracy and reliability was analyzed by comparing the results of three tests. Ninety-six board-certified dermatologists and residents participated and 72 participants completed all three tests. The accuracy and reliability of the assessment of severity components of PASI increased significantly after the educational lecture and the use of reference photographs. Use of reference photographs resulted in limited improvements when the recognition of three-dimensional structures was required, such as in the assessment of thickness or scale. Our study confirmed that the combination of standardized educational training and reference photographs can improve the accuracy and reliability of PASI assessments. Understanding how to evaluate three-dimensional psoriatic lesions can help with proper assessment of the severity of psoriasis.