The hot--dip galvanized transformation induced plasticity (TRIP) steel is an advanced automobile steel with excellent mechanical proprieties, which meets the requirements of energy preservation and environment protection. The microstructure and mechanical properties of hot-dip galvanized TRIP steel are different from conventional TRIP steel treated by continuous annealing processing. Three hot-dip galvanizing processes (main difference is isothermal bainitic transformation (IBT) time) are designed according to the characteristics of hot--dip galvanized TRIP steel manufacturing process to study the influence of microstructure on mechanical properties. By means of SEM, TEM, XRD, dilatometry and tensile test, the microstructure characteristic, phase transformation and mechanical properties were investigated. Combined these experimental results and the carbon diffusion balance theory, the content of martensite and its effect on mechanical properties were analyzed. The results show that the IBT time has a significant impact on the content of martensite. The volume fractions of martensite are 7.08%, 4.17% and 1.18%, when IBT times are 20, 30 and 60 s, respectively. Both the transformation induced plasticity of retained austenite and the martensite hardening are effective but the latter plays a dominant role, when the content of martensite is 7.08\% the strength is similar as dual-phase steel, but a better elongation. When the volume fraction of martensite decrease to 1.18%, the mechanical properties are similar to the conventional TRIP steel, as so small amount of martensite has no obvious effect on the mechanical properties. By changing the hot-dip galvanizing process, the amount of martensite and retained austenite can be controlled, and hot-dip galvanized TRIP steel with different mechanical properties can be obtained.