With selecting earlap as the measuring spot,a multi-layer simulation model of the earlap is established.The transmittance of the earlap model and its distribution regularities under vary beam properties(beam radius,the input energy and its distribution) and model parameters are researched by Monte-Carlo method.Then from the light energy distribution,the detective area and the detective sensitivity of the detector,the input energy needed for certain signal-noise ratio is calculated.It has been proved by research results that the practicable input energy of the earlap can meet the requirement of the acquisition of the dynamic spectrum,higher measurement accuracy of the dynamic spectrum also can meet the spectrum measurement accuracy in the noninvasive measurement of blood compositions.