Simplified models for thermodynamic calculation and indicator diagrams were established based on the operating feature of screw expander．A screw expander system based on organic Rankine cycle (ORC) using R123 as working medium was applied to power generation with associated hot fluid during oil recovery in Huabei Oil Field. After screening main factors, systematic characteristics under variable conditions were attained, by solving new steady state using iteration, analyzing net power, thermal efficiency and energy destructions. The result shows that superheated vapor leads to the decline of output network, although it benefits the output power per unit working medium. The temperature of inlet cooling water affects system performance most and the higher temperature in hot summer results in great deviation of systematic output from design condition. Ensuring the cooling capacity is an effective way to improve the performance. The influence of the change of heat source temperature is larger than that of mass flow．