For a full utilization of the probe after the lunar exploration, the extended mission using the probe for flybys of multi small celestial bodies from the Moon or Earth orbit was proposed. Combined with the flight status of the probe after the lunar exploration mission, the energy requirement for escaping from the Moon or the Earth was emphatically analyzed, together with other necessary conditions for the flyby mission, such as the brightness of small celestial bodies, the capacity of onboard equipment, etc. The methods of selecting small celestial bodies and trajectory design were studied. Aiming at the problems of large search space, large amount of computation and hard optimization of multi target sequences caused by large number of targets, a multi-layer optimal search algorithm for small celestial bodies was designed. Taking Chang′e-5 as an example, the search yielded several feasible solutions for small celestial bodies′ flyby detection. The result shows that the orbiter can approach up to five asteroids under given constraints, including the large Asteroid 12923. The study conclusion can be used for the Chang′e-5 extended mission, and provide beneficial reference for the future missions of the Moon and small celestial bodies.