A robot system with double manipulators used for minimally invasive surgery (MIS) is developed, which can perform operation precisely in the abdomen through cooperation with the instruments. By analyzing the operation environment and characteristics of the MIS, the basic requirements for the mechanical design of manipulator to perform MIS operations are presented. Based on those analysis and requirements, the paper mainly focuses on the mechanical design and optimization of the manipulator. According to the actual requirements of MIS, the manipulator can execute preoperative incision positioning, intraoperative instruments operating and instruments dismounting and replacing. Moreover, the mechanical design of the manipulator can also guarantee the instruments to stay unchangeably at the incision of the patient, which can avoid the non-operative damage to the incision caused by the instrument's operating movements. Furthermore, by using the singular value of the robotics Jacobian matrix, a comprehensive dexterity evaluation index function based on conditional number and operability of the manipulator is proposed, and the dimensional parameters of the manipulator are optimized by adopting the sequential quadratic programming (SQP) algorithm. The optimization results indicate that the manipulator features isotropy and operability, and meets the MIS requirements for commendable mechanical dexterity.