FIR filter is an important type of filter in the field of signal processing because of its inherent stability, simpler structure, and easier implementation on linear phase. WCA is used to solve FIR filter design with low magnitude error and linear phase in passband. This method uses WCA to optimize the filter coefficient based on the low magnitude errors in all frequency domains. Moreover, in the constraint conditions, the least group delay in passband is used for the optimization. The experiments testify this method has good performance on the passband error and group delay error in the passband. Therefore this method is an efficient algorithm for FIR filter optimization which can be used in the digital signal processing widely.