Aimed at the problems that spectrum allocation based on traditional ant colony algorithm in cognitive radio is long in search time and is low in efficiency on pheromone update，a new cognitive radio allocation scheme based on polymorphic ant colony algorithm is proposed in this paper. The scheme changes the regulation mechanism of the single pheromone in the traditional ant colony algorithm, introduces scouts to the algorithm, and marks information in higher benefit value path. And the scheme takes benefit value as a standard to set the index of polymorphic ant colony algorithm. In addition, the polymorphism rule is applied in path selecting and pheromone updating. Finally, Polymorphic Ant Colony Algorithm (PACA) is compared with traditional Ant Colony Algorithm (ACA) on MaxSumReward and MaxProportionalFair. The experiment results show that the proposed spectrum allocation algorithm has a high efficiency and superiority.