To objective study the formation mechanism of magnesium alloy (AZ31) Ttube in the case of 150 ℃, analysis of the internal high pressure forming mechanism of magnesium alloy pipe and finite element simulation are carried out. Internal high pressure forming characteristic in magnesium alloy pipe and lightweight were obtained and realized. The forming pressure was calculated by theory. Then the finite element software Dynaform was used to analyze the effect of different axial feed rate on the forming of the Ttube under the obtained pressure. The effects of different wall thicknesses on the forming under the same internal pressure and axial feed rate were also analyzed. Results show that the optimal forming pressure of magnesium alloy Ttube at 150 ℃ is 20 MPa, axial velocity is 5 m/s and wall thickness is 1 mm. It is found that the formation of magnesium alloy requires proper coordination in temperature, internal pressure and axial thrust.