Larval instars of Tetramoera schistaceana and its developmental duration were clarified to study its biological and ecological characteristics, providing scientific basis for forecasting and controlling this pest. T. schistaceana fed with natural feed Zizania latifolia (Griseb.) in the laboratory were the research object in this paper. Head capsule widths and developmental periods of larvae at each larval instar were measured and analyzed by Crosby ratio, Brook ratio, frequency distribution and regression analysis. The results showed that T. schistaceana passed through five larval instars to attain its pupal stage where the head capsule widths of 1^st~5^th instar larvae were 0.293 mm, 0.437 mm, 0.615 mm, 0.911 mm and 1.222 mm, respectively. Their head capsule widths conformed to Dyar's law and Crosby's law. The fitting degrees of linear, quadratic and exponential fitting models between head capsule width and instar were all fine. When the coefficient of determination (R²) of the exponential fitting model was the largest, the fitting degree was the best. So head capsule widths could be used as an important indicator for larval instars determination. The mean instars of 1^st~5^th instar larvae were 2.766 d, 2.913 d, 3.107 d, 3.413 d, and 8.860 d respectively at 26℃. T. schistaceana larvae had five instars. The head capsule width was an important index for distinguishing the larval instar of T. schistaceana. From egg hatching to pupation, female and male larvae needed 20.265 d and 19.250 d on average at 26℃, respectively.