1. A safety monitoring method for human-machine symbiosis, comprising: a spatial image capturing step for acquiring at least two skeleton images; an image recognition step for generating at least two spatial gesture images corresponding to the at least two skeleton images, based on information of changes in position of the at least two skeleton images with respect to time; a human-robot-interaction safety monitoring step for generating a gesture distribution based on the at least two spatial gesture images and a safety distance; and a process monitoring step for determining whether the gesture distribution meets a safety criterion; wherein the image recognition step comprises: acquiring images for recognizing a human skeleton, a human skeleton dilation region, a robot skeleton, and a robot skeleton dilation region, to generate the at least two spatial gesture images.