One of the recent trends in radiation therapy is to increase conformal and accurate dose delivery such as in stereotactic radiosurgery (SRS). Treating small lesions and brain disorders requires the accurate placement of small radiation fields deep inside the human cranium. To design a collimator meeting these requirements, a new numerical concept was developed, which is presented here. The algorithm proposed here can generate beam profiles of plural collimation apertures and arbitrary initial beam spot distributions in a time-efficient method. It is an ideal tool to optimize collimator design for penumbra, dose rate, and field size. The intensity of the source beam spot is divided into slices, and each slice is projected onto the treatment plane at the isocenter through the collimator apertures. The illuminated field range and intensity are determined by geometry and the intensity of that slice of beam source, respectively. By integrating the projected intensity across all the slices of the source profile, the profile on the treatment plane is obtained. The algorithm is used to generate beam profiles of a conical pencil beam collimator system and compare them to the Monte Carlo simulation as well as measurements. It can also be used to demonstrate the impact of collimator shape on the beam penumbra, dose rate, and field size. The projection integration method provides a quick and informative tool for collimator design. The results were validated with the Monte Carlo simulation and measurements. This method was demonstrated to be effective for optimizing beam characteristics.