The invention provides a preparation method of a light absorption layer of a CIGS-base thin film solar cell. The method comprises the following steps that: a substrate is provided, and a prefabricated layer is deposited on the substrate; an inert gas, a selenium-containing object and a sulfur-containing object are introduced into a heating furnace, and heat energy is transferred into the furnace, and temperature is increased to first temperature from room temperature, and keeps constant for a certain period at the first temperature; the temperature is increased to second temperature from the first temperature, and keeps constant for a certain period at the second temperature; the temperature is increased to third temperature from the second temperature, and keeps constant for a certain period at the third second temperature; the temperature is decreased to fourth temperature from the third temperature, and keeps constant for a certain period at the fourth second temperature; and the temperature is decreased to room temperature. With the preparation method of the invention adopted, the light absorption layer can have larger grains, and therefore, higher open-circuit voltage and a high fill factor can be realized.