Abstract: Interest in passive radar has risen up in resent years for the following advantages of this kind of radar, such as cover ability, anti-stealth, antijamming capability, simple structure, low cost and so on. Selection of the illuminator is one of the critical technologies. First, the advantages and disadvantages of varies of spaceborne illuminator was discussed, and the superiority of Beidou illuminator was attained by comparing Beidou and other GNSS. And then the detection ability and the single/bistatic ambiguity were analyzed. The result shows that Beidou satellites are more suitable as opportunity illuminators in China