Quasi-static compression, tension and combined shear-compression tests were conducted to characterize the mechanical behavior of PMMA. The cylinder samples and dog-bone samples were used for uniaxial testing. The shear-compression samples of four different angles (15 degrees, 30 degrees, 45 degrees, and 50 degrees) were used to obtain the multiaxial stress state instead of complex loading equipment. The yield behaviors of PMMA cannot be described by the Tresca or Mises criterion due to the effects of both the first invariant of the stress and the third invariant of deviatoric stress. A criterion is proposed that includes the first invariant of the stress and the third invariant of deviatoric stress in this paper. In other word, the effects of hydrostatic pressure and Lode angle are considered. The proposed yield criterion has the capability to precisely capture the experimental yield loci of PMMA.