Nano gold refers to the particles with size between 1~100 nm, it can be used to strengthen the light absorption of blood in the process of laser treatment of vascular dermatosis because of its good biological compatibility, structural stability and optical properties. The gold colloid is prepared with sodium citrate as reducing agent. Effect of temperature, reaction time, sodium citrate dose on the size and absorption spectra of gold colloid is studied. Thus the best condition for preparing gold colloid is obtained. After the gold colloid with size of 16 nm is prepared, it is added to blood to study the effect of colloidal gold on the properties of light absorption of blood as a function of temperature. The results show that the absorption of blood increases with the increase of volume ratio. Blood absorption peak values at 545 nm and 581 nm increase by 1.33 times and 1.09 times respectively when the ratio is 5. Under the condition of variable temperatures, colloidal gold does not affect the dynamic absorption characteristics of blood.