In recent years, collision safety between a human and a robot has been increasingly important along with the spread of service robots. Much research has been conducted on how to design a safe manipulator, but it is not applicable to real manipulators yet. To deal with this problem, we propose a guideline to design a safe manipulator, which is composed of two phases, the safety evaluation and design parameter adjustment. To evaluate collision safety, we adopt the effective mass and manipulability and simplify the spatial collision between a human and a multi-DOF manipulator. Moreover, the experimental results from the biomechanical literature are employed for more realistic evaluation of collision safety. The design parameters and proper adjustment strategy can be established based on the safety evaluation results, and the design parameters of a manipulator can be modified to satisfy the collision safety and design requirement. The proposed guideline is implemented in the design of a 3 DOF safe manipulator, and it is shown that this guideline can be used to design a multi-DOF safe manipulator.