Abstract: This paper discusses about the trade-off between detection probability, false alarm probability and observation period for the broadband spectrum energy detection. By utilizing the threshold based on the minimum-error-rate criterion, the closed-form expression of the minimum watching time for the detection is derived, which can make the detection satisfy the performance constraints. Besides, the effect of noise uncertainty is also considered in the detection and the expression of watching period with peak noise uncertainty is provided. All conclusions are verified by the numerical simulations.