In order to solve the problem of fast differential current instantaneous trip protection caused by current transformer saturation， a fast differential current instantaneous trip protection algorithm based on phaselet algorithm is developed， which can rapidly complete fault detection. The linear relationship between phaselet algorithm using sampling data of one-fourth period and one period Fourier algorithm is studied， and then the linear relationship matrices of the two algorithms are derived. The amplitude-frequency characteristics of phaselet algorithm are analyzed and the fast differential current instantaneous trip protection criteria are proposed. The static test and dynamic test validate the feasibility of the proposed algorithm and protection criteria.