In the actual operation of the anaerobic reactor UASB, the water distributor at the bottom of it is often blocked, resulting in uneven water distribution. In order to improve the treatment effect, according to the characteristics of the water distributor and the hydraulic calculation principle of the annular pipe network, and considering the local resistance that must be involved in the water distributor, the finite element method is used to carry out hydraulic calculation, and the shortcomings in the structure size are pointed out. Application examples show that the calculation results are reasonable and can provide a certain design basis for water treatment engineering.