The parameters of water inlet conduit of the time-accumulating sediment sampler are one of the key factors for the flow rate at water inlet. For the sake of reality and accuracy, it is necessary to make the flow rate at water inlet of conduit approximately equaling to that of water in river. Based on this, a mathematical model for the flow rate of sample water in conduit was built, and the dimensions of the conduit was optimized. After optimization, it shows that the possibility of coefficient K is 98% within the range of [0.9, 1.1 ] when the water rate in river is within the range of 0-5 m/s. By using the optimization results, a new sediment sampler was developed. The test results show that the flow rate at the water is equal to that in river in the condition that the water rate is under 0-5 m/s.