Objective: To develop the Assessment of Smartphone Multitasking for Adolescents (ASMA) and evaluate its reliability and construct validity, so that to provide an evaluation tool for relevant epidemiological studies. Methods: Based on literature review, expert based judgement, and group discussion, the self administered ASMA identified 3 dimensions named smartphone activities and non-media activities multitasking, smartphone activities and other media activities multitasking, and smartphone functional use multitasking, which initially included 27 items. From October to December 2021, 5 566 college students were selected from 7 regions, including Liaoning, Shanxi, Henan, Shanghai, Chongqing, Shenzhen, and Yunnan by multi stage cluster sampling method, and item analysis, factor analysis, and reliability evaluation were used to screen items and evaluate reliability and validity. Results: The final version of AMSA contained 3 dimensions covering 26 items. The variance cumulative contribution rate was 59.63 %. The internal consistency test showed that Cronbach s alpha coefficient for the questionnaire was 0.944, and ranged from 0.838 to 0.928 for each dimension. The split half coefficient of the questionnaire was 0.849 and ranging from 0.781 to 0.874 for each dimension. The results of confirmatory factor analysis showed that the root mean square error of approximation (RMSEA), comparative fit index (CFI), and Tucker Lewis index (TLI) of the questionnaire were 0.068, 0.901, and 0.891, respectively, which had which had a good fitting degree. The results of Pearson correlation analysis showed that the criterion validity of the questionnaire was good ( r=0.206-0.351, P <0.01). Conclusion The ASMA is consistent with the evaluation standard of psychometrics and can be used as an assessment tool to evaluate smartphone multitasking behaviors in adolescents.