Effect of rotational motion of porphyrin molecules on the luminescence dynamic of low-doped porphyrin side-chain polymers has been studied by using the theory of exciton rotational diffusion. It is shown that the rotational motion of porphyrin molecules is an important way that induces the nonradiative relaxation of excitons. The more easily porphyrins rotate， the more rapidly energy decays， therefore， the faster the luminescence decay process. In the case of low doping concentration of porphyrins and long interchain distance， the rotational motion of porphyrin molecules is the main reason of fast luminescence decay and low luminescence efficiency. The transient luminescence relaxation processes of two kinds of polymers were fitted with the theoretical expression， the theoretical analysis are in good agreement with the experimental results.