Abstract: Based on the analysis of primary users spectrum-usage characteristics, an asynchronous sensing period optimal model on minimal costs of channels is presented, and an Adaptive Fuzzy Adjustment Algorithm (AFAA) of spectrum sensing period is proposed for time-varying spectrum-usage characteristics. The AFAA can adjust sensing period adaptively in real time using fuzzy logic with parameters optimization. Experimental results show that the AFAA is effective under many kinds of situations where spectrum-usage probability distributives as well as the change rates of these distributive parameters are different, and has better stability when the variation of distributive parameters is high.