Aiming at the problem that conventional airspace search method has a low effectiveness in radar EW reconnaissance, a dwell time adjusting method based on the minimum description length criterion (MDL-ADT) is proposed.The Digital Beam-Forming (DBF) technology is introduced in airspace search, and the relationship between the intercept probability of radar reconnaissance receiver and the beam dwell time is analyzed.The method has two steps:1) Pre-searching, using the MDL criterion to estimate the source quantity in each beam;and 2) Allocating the dwell time according to the distribution of signal sources in airspace, and implementing airspace search.The method ensures most efficient utilization of the resources of EW reconnaissance system by reasonable distributing of the dwell time.Finally, a simulation example shows that the proposed method can improve the airspace searching effectiveness by 20%.