Abstract: During Instrumentation Channel Calibration Test（T1 test） or under the condition of instrumentation malfunction,mistakes due to man-made intervention are observed now and then,so that the steady state operation of the unit is not guaranteed.In order to solve this problem,control logic of Second Highest Selection,Second Lowest Selection has been introduced to the control system of four units in the Second Plant of CNNP Nuclear Power Operations Management Co.,LTD.This paper describes the operation principle,method of work and detailed alteration of Second Highest Selection,Second Lowest Selection.Besides,based on the analysis of test data,the feasibility of this alteration project has been validated.At the same time,this paper introduces the detailed application of this alteration.