For quite some time little attention has been given to the measuring accuracy of the fault recorder， which has led to the insufficient accuracy of the results derived in various applications based on the recorded data， weakening the due reference value of the recorded data. Because of the linear and non-linear errors existing in the analog sampling circuit， the relationship between input and output is not a purely linear but a non-linear expression. The non-linearity has a great influence on the measuring accuracy， so the fault recorder will generate larger errors when the linear expression is used to represent the input-output relationship. In order to reduce the impact of non-linearity， the best fitting curve and segmental best fitting curve are proposed to replace the linear expression. The test results on an actual fault recorder show that the proposed approach can greatly improve the measuring accuracy even in a very large measuring range.