For the existing sampled value protection, the fuzzy operating zone is too large and the protection settings are difficult to set for users. To solve the above problem, a new algorithm on the sampled value protection based on quadratic interpolation algorithm is proposed. The new algorithm generates virtual zero crossing sampled values based on interpolation algorithm. Within the selected data window, the ratio of meeting the operation conditions time and data window time is calculated, which will be compared with an inner setting. The comparison result is used as a main criterion of the sampled value protection based on interpolation algorithm. The algorithm can greatly improve the accuracy of operating threshold and eliminate the effects of fuzzy operating zone without increasing the sampling rate of computer protection devices. The results of computer simulation and prototype testing demonstrate the validity of the proposed algorithm.