In order to clarify the optimal liquid-solid ratio (L/S) values for the activation of metakaolin at different concentrations of phosphoric acid solution, five sets of phosphoric acid solution of 4, 5, 6, 7 and 8 mol·L^-1 were set up, and four L/S values of 0.6, 0.7, 0.8 and 0.9 were used for preparation of metakaolin-phosphoric acid based geopolymer (MPAG). The 7 and 28 d compressive and flexural strengths of MPAG were used as the evaluation indexes to investigate the effects of above preset variables on its mechanical properties. The compressive strength of samples corresponding to the optimal L/S value concentration of each group after sulfate erosion were further investigated. Finally, the phase composition and microscopic morphology of MPAG were characterized and analyzed. The results show that MPAG is mainly composed of an amorphous aluminum phosphate phase. Greater compressive strength can be obtained by using L/S value of 0.6 when the concentration of phosphoric acid solution is 4 ～ 6 mol·L^-1, and when the concentration increase to 7 ～ 8 mol·L^-1, the recommended of L/S value is 0.7. MPAG exhibits good sulfate resistance, with a compressive strength retention rate of 84% to 93% after 28 d immersion. The samples prepared with 6 mol·L^-1 phosphoric acid under any L/S value achieved the maximum flexural strength for that L/S value. The 28 d maximum flexural strength is 6.80 MPa when L/S value is 0.6. The results of this study can provide references for the preparation and application of MPAG.