We find that the increasing coupling strength can lead to a decreasing density of power inside the waveguide of a photonic crystal waveguide (PC-WG) directional coupler, which is called the mode power remaining phenomenon.This phenomenon is detrimental to achieving low insertion loss of the coupler. An improved structure of the PCWG directional coupler is proposed by simply increasing the radii of air holes in the post coupling region. The simulation results demonstrate that the insertion loss can be reduced dramatically by suppressing the remained power, and therefore both the short coupling length (8 μm) and low insertion loss (lower than 0.5 dB) can be obtained.