The invention discloses a method for cultivating pleurotus eryngii uniformly rich in selenium with a mixed organic selenium source. The method is characterized in that the method comprises the steps of mixing selenium-rich cultivating material additive accounting for 1-10 percent of the total weight of dried cultivating material into the dried cultivating material, and cultivating pleurotus eryngii according to an ordinary cultivation method to obtain pleurotus eryngii uniformly rich in selenium; the selenium-rich cultivating material additive has a total selenium content of 5-50 mg/kg, and needs to be crushed first and then compressed to form particles of certain grain sizes, the grain sizes of the particles in the selenium-rich cultivating material additive are in the range of 0.005-5.0 mm with multi-grain-size regional distribution, the particles having a grain size of 1.0-5.0 mm account for about 10 weight percent of the total weight of the additive, while the particles having a grain size of 0.005-0.1 mm account for at least 40 weight percent of the total weight of the additive. The method has the advantages that the selenium utilization factor of the cultivating material in pleurotus eryngii is high, the selenium content in different batches of picked pleurotus eryngii is stable, therefore, the method is highly safe, convenient to operate, and easy to promote.