Theoretically， the resistance， inductance and capacitance in per unit length， and the effective length of transmission line cannot be obtained accurately. Influenced by many factors， these parameters acquired by practically measurement are only the approximate values. To two-terminal fault location， the parameter deviation of the settings from actual values will have an adverse influence on the accuracy of fault location. By deducing the fault location function of parallel transmission lines， the mechanism that parameter deviations influence the locating accuracy is achieved. In the paper， the improving measures are also given to locate faults in the situation of parameter deviation. Simulations show the validity of the proposed methods.