Abstract: The reproductive biology of Carassius auratus gibelio was studied with 546 fish collected from the lower reaches of the Irtysh River in Xinjiang Uygur Autonomous Region from April to October 2013. The overall sex ratio (F/M) was 10.84:1. The development of gonads can be divided into six periods according to their histological characteristics. The monthly variation in the proportion of gonads at each macroscopic maturity stage and gonadosomatic index (GSI) indicated that C. auratus gibelio spawns once a year and that its spawning season is from May to July, with peak spawning occurring in June. The estimated standard lengths (SL₅₀) and ages (A₅₀) at first sexual maturity for females and males were 161 and 135 mm and 2.3 and 1.9 years, respectively. The absolute fecundity (AF) of C. auratus gibelio was 42453 eggs per fish, and the relative fecundity (RF) was 98.19 eggs per gram of body weight (BW). The absolute fecundity had a linear relationship with body length and a power function relationship with weight but no significant correlation with age or ovary weight. This study further enriches the biological data available for C. auratus gibelio and provides a scientific basis for the protection and sustainable utilization of C. auratus gibelio resources in the Irtysh River.