As the existing ray tracing algorithms exhibit low efficiency in indoor scenarios, this paper presents a novel ray tracing approach for computing the electromagnetic field within a building. Firstly, a matrix is incorporated to denote the visibility of all surfaces. Among all candidate rays between a pair of source and field points, the least likely ones are eliminated by the visibility matrix. The remaining rays are then analyzed using the conjugate gradient method to precisely determine the ray path. Subsequently, ray-object intersection tests are carried out, which are also expedited by the visibility matrix. Eventually, if the ray is present, the electromagnetic field is calculated via the uniform theory of diffraction (UTD). This new ray tracing algorithm can handle all types of rays, such as the reflected, diffracted, and refracted ones. Hence, it is more flexible compared to the image method. An example of a house demonstrates that it is more accurate and faster than WinProp in indoor scenarios.