X-ray detector has experienced the era of the gas detector and the scintillator detector, and entered the era of semiconductor detector. As a new type of a semiconductor detector, the silicon pixel detector has the characteristics of high resolution, high detection efficiency, fast time response and low power consumption. In order to study the detection efficiency of silicon pixel detector, an X-ray detection system is designed and the experimental research is conducted. Four kinds of X-ray energies of 4.51, 5.41, 6.40, 8.05 keV are tested. After error analysis and data processing, the detection efficiencies of frontal incidence are 53.00%, 51.56%, 40.65% and 29.91%, respectively. This experimental study provides a new approach for finding a X-ray detector with high detection efficiency .