In this paper, we discuss and propose how to analyze and synthesize the timbre of stringed mzlsical instruments, especially electric bass guitars and pianos. The structure of harmonics' elements on the Fourier spectrum is investigated. And a novel method is proposed to deform the structure while keeping the same timb1.e for monophonic instrument sound, that is, what kind of restriction must be imposed. To coiz~?m the eflectiveness of our proposed method, experiments are conducted and the results are shown. According to the rule introduced here, ir is possible to synthesize the new timbre belonging to a stringed instrument.