A surgery assistive system includes an instrument having a tool and a manipulator connected to the tool, a spatial sensor system for detecting spatial information of the tool, and a computer system for manipulating a kinematic state of the manipulator. A method for obtaining surface information by the surgery assistive system includes the steps of: defining a target region and a plurality of reference points on a virtual anatomical model of a subject； prompting a user to generate sampling information by using the instrument to sample a plurality of sampling points on the subject corresponding to the reference points； and designating the sampling information as surface information of the sampling points. Each piece of the sampling information includes a coordinate of one of the sampling points, an angle of a contact of the tool at the sampling point, and parameters associated with the contact.
