Abstract: A space-time two-dimensional spectrum sensing method which used mean detection probability constraint was put forward for spectrum holes detection in cognitive radio networks. The constraint for secondary user to access the spectrum of primary user was satisfied by deriving the mean detection probability within the whole detection range, based on which the detection threshold was set to derive the spectrum holes accessing probability in time, space and two dimensions. Analysis and numerical results show that the false alarm probabilities in time and space dimensions are both decreased when using the mean detection probability to satisfy the constraint instead of using the detection probability, and the spectrum holes accessing probability in the cognitive radio networks is greatly increased.