In this study, one type of ternary composite catalysts including tetraisopropyl titanate (TPT), p-toluenesulfonic acid (TSA) and 2,2-dihydroxy propionic acid (DMPA) was developed for polymerization of lactic acid (LA). The activity of central metal titanium was efficiently adjusted by the interaction between DMPA and TPT. During the process of LA polymerization, TSA instantaneously occupied the vacant orbital, which efficiently decreased the side reactions of the polymerization. DMPA which complexed with Ti participated into the polymerization of LA, benefiting to significant enhancement of the final molecular weight. Under the catalysis of TPT/TSA/DMPA with optimal mole ratio （1∶3∶5）, the average molecular weight of PLA is over 20×10⁴. The structures of complex catalysis and the polymer were measured by ¹H-NMR, ¹³C-NMR and DEPT 135, and the probable intermediate of polymerization was deduced. This ternary composite catalyst provides one promising to realize synthesis of PLA with high molecular weight by melt polymerization.