This experiment studies the light absorption laws of the blood componentS between 240 to 800 nm. The results show: 1) The absorbance and transmittance laws resemble each other in blood of Group A, B, AB and O. 2) Between 600 to 800 nm, absorbances of the whole blood, erythrocyte, leukocyte, plasma and serum are less than 5 percent, while the transmittances of them are more than 95 percent. 3) To erythrocyte and lymphocyte, typical absorption peaks appear at (416.57±1.90) nm, (542.71±1.80) nm, (578.57±1.81) nm.