In recent years the collision safety between a human and a robot has been increasingly important because of the spread of service robots. In order to design a safe robot arm, the collision safety evaluation must be conducted prior to the construction of the robot arm to compute the necessary design parameters. Previous evaluation methods required the use of the actual robot, which are both time consuming and expensive. In this study we propose a new human-robot collision model and a collision safety evaluation method which does not require the use of the actual robot. A human-robot collision model is developed, and the collision safety of a 3 DOF planar robot arm is evaluated. Then, using the evaluation results, the design of the robot arm is modified to ensure the collision safety. The proposed evaluation method enables the appropriate design parameters for a safe robot arm to be determined in a short period of time at the minimal cost.