Approximate entropy (ApEn) was developed to quantify the effects of welding parameters on the process stability of narrow gap laser-arc hybrid welding (NGHW). The smaller the ApEn of arc current, the higher the welding stability. A dimensionless parameter, eta (A), which was the ApEn increment of the NGHW to the hybrid welding without narrow gap, was proposed to quantitatively characterize the effects of space constraint on the process stability. All the eta (A) were negative. It had a good agreement with the experimental values and demonstrated that the process could be stabilized by space constraint. The eta (A) was minimized to be -16.7%, but usually at the range from -8 to -1%. Increasing arc current, laser-arc distance, and welding speed, or decreasing laser power, all increased the eta (A) by weakening the space constraint. Finally, the stabilization mechanism was discussed in terms of the effects of space constraint on the behaviors of laser keyhole and arc droplet transfer.