Signals transmitting in the multipath channels are usually disturbed by multipath interference. Therefore, the signal received in the receiver deforms, and the communication quality in the channel slumps dramatically. In this paper, one kind of optimization method involving in spectrum shaping of cognitive signal is proposed by comparison with the signal without spectrum shaping, according to the related knowledge of information theory. It can be justified theoretically that the new approach improves the information transmission rate and communication quality significantly in the practical communication channels. Besides，the simulation result further confirms that the method of spectrum shaping plays an important role in improving the quality and transmission rate of cognitive signal in the multipath channel.