The invention discloses a testing device and a testing method for optical uniformity of a biaxial crystal in a principal axis direction. The testing device comprises a light source, a condensing lens,a collimating lens, a polarizer, a phase compensator, an analyzer, an imaging lens and a camera which are sequentially arranged in the direction of an optical axis, wherein a crystal to be tested isarranged between the phase compensator and the analyzer, the phase compensator is composed of two optical wedges, phase retardation is adjusted by adjusting the relative positions of the two optical wedges, and the optical wedges are made of crystals of which components are the same as that of the crystal to be tested and have good optical uniformity. The testing device is simple in structure, lowin cost and convenient to maintain. Compared with a traditional testing method, the testing method has the advantages that the testing method does not need to test background interference and interferograms formed by lasers in different polarization states passing through the crystal repeatedly, is little affected by the environment, has high test efficiency and is suitable for batch production and detection; and the method is simple to operate, has low processing requirements on the tested crystal, and is easy to popularize and use.