The invention discloses a method for drawing unbiased photon mapping in the presence of a medium. The method improves the unbiased photon gathering method from aspects such as improving subpath sampling technology, reforming theory formula, sampling importance of unbiased item estimation, and discussing medium boundary conditions, etc., which enable continuous obtaining of unbiased drawing results in the scenario of the presence of the medium. According to the invention, improvement is made to the weight of multiple importance sampling so as to combine the method for drawing unbiased photon mapping in the presence of the medium with a bidirectional light ray tracking technology, which enables the two methods to simultaneously exhibit advantages under different contexts.