Aiming at the problem that the application of IEC 61784-3 residual error rate extended calculation model reference calculation formula is greatly limited, the corresponding improvement suggestions are put forward. Taking openSAFETY as the object of analysis, combined with its operation mechanism, after thoroughly analyzing the principle of reference formula in extended computing model, this paper puts forward suggestions for improving the calculation formula in extended computing model of residual error rate, and completes the security integrity evaluation of openSAFETY under IEC 61784-3 extended computing model according to the improved formula. The results show that the improved extended model is more applicable and accurate than the original model in calculating the residual error rate of openSAFETY.