Abstract: In this paper, we construct a simple but reliable ecosystem health indicator: Aquatic Animal Species Index (AASI). The assessment parameters are species numbers (S_O) of fish, benthos, etc., and resource measures (R_O) of important groups; the reference values (S_I, R_I) are the highest achievable levels of allied water bodies or those of the first year of assessment. AASI of one group (SS_O/I) equals W_S S_O/I+W_R R_O/I, where S_O/I is Species Number Index, R_O/I is Resource Index, W_S & W_R are the weight, i.e., 0.7, 0.3, respectively. AASI of several groups (ΣSS_O/I) is calculated as the mean of SS_O/I. If reference values are those of the first year of assessment, the health assessment is based upon index increments, and the highest levels can be ascertained after monitoring for many years. Using benthos data of lakes along the Yangtze River and the mainstem, we demonstrate the effectiveness of AASI. Compared with multi-metric indices and RIVPACS-type models, the AASI method is simpler and easier, so can be applied widely.