Abstract: This paper presents a timbre model of computer musical instruments based on the trapezoid wave，in order to describe the timbre of the computer musical instruments，to construct the software musical instruments，and to make it possible that the instrument timbre can be controlled by modulating the parameters.This model includes two sub models，the vibration model and the amplitude envelope model.This paper makes the parameter values of the amplitude envelope invariable during analyzing the model，defines the timbre parameters，including the time coefficient，the unsymmetrical degree，the trapezoid degree and the triangle degree，and gets the frequency spectrums of the model in different parameters using one dimensional DCT in VC++ environment.This paper also finds the rules of the parameters acting on the instrument timbre，by analyzing how the timbre parameters influence on the frequency spectrums.The analysis shows that the base tone is always the strongest one in the frequency spectrums，and there are no even harmonic waves if it is an isosceles trapezoid.Furthermore，the high frequency components are rich when the unsymmetrical degrees away from 1，the trapezoid degrees，or the triangle degrees away from 0.25，are great；thus the instrument timbre is bright，contrarily the timbre is tedious.So it is important practical value and fundamental significance for the timbre composition of computer musical instruments and algorithmic musical composition.