The rate of interturn fault on the transformer part of controllable shunt reactor of transformer type is high， whereas the sensitivity of protection is low. In order to solve this problem， first， the mechanism of interturn fault of transformer type reactors is analyzed， then different variation laws of leakage inductance on each side and the equivalent total leakage inductance of main reactor under normal and other operations is discussed. Based on the percentage restrain characteristic， a novel interturn fault protection principle is proposed， in which the standard deviation representing variation of the equivalent total leakage inductance is taken as action component， while the stability of variation of each side leakage inductance is taken as restraint component. That is the way to structure new protection criterion of percentage restraint characteristic. When interturn fault occurs， the action component increases and the restraint component decreases， so the new criterion can operate correctly. In contrast， the action component decreases and the restraint component increases under normal operation， external fault and etc， the new criterion will not mal-operate. Lots of simulation verify good effectiveness of the new criterion.