Background Subjective cognitive decline-questionnaire 9 (SCD-Q9) was developed to detect SCD complaints at risk of mild cognitive impairment (MCI). However, our previous findings indicated that its coverage might be insufficient. To test this hypothesis, we recently translated SCD-Q21. Objective To examine the reliability and validity of this translated SCD-Q21 and to explore its effectiveness for discriminating MCI from controls. Methods Item analysis was performed to understand its item discrimination and homogeneity. The Cronbach's alpha and Spearman-Brown's split-half coefficients were calculated to test its reliability. The Kaiser-Meyer-Olkin (KMO) value, Bartlett's sphericity test, and exploratory factor analysis (EFA) were used to examine its construct validity. The content validity was evaluated using five-grade Likert scale. Finally, the SCD-Q21 scores in MCI and controls were compared. Results The difference of each item between the extreme groups was significant. The Cronbach's alpha coefficient was .913 and Spearman-Brown's split-half coefficient was .894. When performing holding one-out approach, the Cronbach's alpha coefficient ranged from .906 to .914. The KMO value was .929 and the difference of Bartlett's Sphericity test was significant. All experts scored 5 points when assessing its content. Finally, a significant difference of score was found between MCI and NC groups. Conclusions The reliability and validity of the SCD-Q21 are good, which may pave a way for its application in a wider Chinese-speaking population.