The present invention discloses a mutant KCNQ1, namely S140G-KCNQ1. It also discloses a method for screening inhibitors of the cardiac KCNQ1 potassium channel using KCNQ1, which includes the steps: (a) introducing into mammalian cells (i) an expression vector for KCNQ1 and (ii) either a KCNE1 expression vector or a KCNE2 expression vector; (b) in the culture medium of the transformed mammalian cells, adding a candidate substance and measuring the electrophysiological potassium ion flow before and after addition, wherein if the electrophysiological potassium ion flow decreases after adding the candidate substance, it indicates that the substance is an inhibitor of the cardiac KCNQ1 potassium channel. This method can rapidly screen candidate drugs for the treatment of atrial fibrillation.