Due to the small mode field of silicon photonic chips, the large coupling loss is a road-blocker for their applications. In this work, the coupling efficiency between a silicon photonic modulator chip and a single-mode optical fiber is investigated. The mode field matching theory is analyzed at first. Then the mode field test of silicon chip and single-mode optical fiber for coupling is carried out. Next, a low-cost coupling method to improve coupling efficiency is proposed. The experimental results show that the coupling efficiency of the single-mode optical fiber and silicon photonic chip reaches 70%. The optical module is made by using this scheme, and the 4 Pulse Amplitude Modulation (PAM4) eye diagram is tested. The experimental results show that better eye diagram can be obtained.