A differential game guidance law for bearings-only measurements is proposed to improve the estimation performance of the missile guidance information. The situation of line of sight angles only or bearings-only measurements can be generated from disturbed or passive strapdown seeker of missile. First, through the analysis of the game space of the norm differential game guidance law, it is clear that the characters of the capture zone can be used to improve the observability of bearings-only measurements. Secondly, on the basis of completing the design of bearings-only estimator, the relationship between missile commands and observability is established by using the character of estimation mean square error reflecting the observability. Then, in the capture zone of the differential game space, guidance control commands are selected to guarantee the miss distance and improve the observability. Lastly, the execution process of the proposed differential game guidance law for enhancing observability is presented, and the zero-effort miss distance parameter and the time-to-go parameter are designed for the consideration of the control system delay, principle of miss distance control priority and other external uncertainties. This approach of guidance law design breaks through the frame of certainty equivalence principle, and realizes the integrated design of filter and guidance law. The nonlinear simulation comparison shows that the proposed guidance law can improve estimation performance of the target information and has higher single-shot kill probability.