The spectral time evolution of optical modes in planar random media is analyzed by using the finite difference time domain method to numerically solve Maxwell equations. The character of random media can be described by some factors, including the random constitution of scattering particles, medium parameters (such as the surface-filling fraction and the size of particle), and the shape of me dium. Such factors determine the frequency property and number of optical modes in random medium under quasi-stable state. However, the media with different f actors have approximately the same character and rate for the generation, sele ction and evolution of optical modes. Such cold cavity properties of planar rand om media are similar to that in conventional optical cavities.