Objective To design and validate a knowledge, attitude and practice (KAP) questionnaire on disclosure of information to patients among medical staff, and to provide reference for assessment and intervention. Methods Based on the KAP theory, a draft questionnaire was formed through literature review, qualitative interview, group discussion, Delphi expert consultation, and pilot test. A total of 438 clinical medical staff were surveyed to test the reliability and validity of the questionnaire. Results A final questionnaire was produced with 31 items on 3 dimensions: knowledge, attitude and practice.Exploratory factor analysis identified 3 common factors, which could explain 71.906% of the total variance.Confirmatory factor analysis indicated the 3-factor model best matched the data: χ2/df=3.108,RMSEA=0.098,RMR=0.023,IFI=0.924, and CFI=0.924. The Cronbach′s alpha coefficient of the total questionnaire was 0.948 and the test-retest reliability was 0.973. The scale-level content validity index was 0.984, and the item-level CVI ranged from 0.860 to 1.000. Conclusion The questionnaire is reliable and valid for assessing medical staff′s KAP of disclosure of information to patients.