Near infrared spectroscopy (NIRS) has advantages such as high speed, no destroying the samples, and online analysis. Therefore, NIRS is one of the most prospective ways to be applied in noninvasive biochemical analysis of blood. However, the absorption spectrum of blood is weak, and in near infrared noninvasive biochemical analysis, light energy utilization efficiency of conventional spectral instruments is lower. Thus, it is a little difficult in clinical noninvasive analysis with NIRS. In order to solve this problem, ellipsoidal reflector is used which has the focusing characteristic. The distributions of image points are analyzed according to geometric optics, and initial structure of ellipsoidal reflector is optimized by the method of ray tracing. In this way, the detector receives diffused light of finger which is collected with high efficiency. Through the simulation, light energy utilization efficiency of this ellipsoidal reflector raises nearly five times than that of non-optical collection device. It is beneficial to test the absorption spectra of blood and improve the signal-to-noise ratio(SNR).