The invention provides an electrolytic polishing agent and an electrolytic polishing method for pretreatment of magnesium alloy residual stress test. The electrolytic polishing agent comprises phosphoric acid, glycerinum and absolute ethyl alcohol according to the volume ratio of 1:1-2:2-3. With the electrolytic polishing agent, residual stress test result accuracy is improved, operation is simplified and polishing effect is stable. Compared with the prior art, the electrolytic polishing agent has the advantages of wide source of raw materials, preparation simplicity and low price. Besides, the electrolytic polishing method for pretreatment of magnesium alloy residual stress test is easy to operate and capable of realizing magnesium alloy polishing layer by layer with short treatment time and stable treatment effect.