In order to study the variation of mechanical behavior and internal pore structure of concrete under sulfate attack, uniaxial compression tests were carried out on concrete specimens soaked in 0%, 10%, 15% and 20% (mass fraction) sodium sulfate solution for 210 d. The stress field distribution was simulated by PFC2D software to study the crack evolution during the test, and the internal pore distribution was analyzed by nuclear magnetic resonance. The test results show that the peak stress and elastic modulus of the concrete decrease with the increase of immersion concentration. According to the PFC2D simulation results, tensile cracks first appear in the concrete during uniaxial compression, then the cracks continue to expand, and tensile and shear composite cracks appear, finally a shear crack runs through the diagonal. The T2 distribution curve of concrete after sulfate attack shows three peaks, and the internal pores are mainly micro pores and small pores. With the increase of immersion concentration, the micro pores gradually transform into small pores, resulting in the increase of T2 peak 1 amplitude and area of the concrete.