The problem of observability for bearings-only target tracking is discussed in this paper based on the target traveling in the three-dimensional space at a uniform velocity, constant acceleration and first-order lag acceleration. Pseudo measurement equations are constructed, Grammian criterion of linear system theory is used, and the problem of solving complex nonlinear differential equations is avoided, the method can be used to deal with the general situation, and is not restricted to Nth order target dynamics. The analysis of the necessary and sufficient conditions shows that some types of observer maneuver do not satisfy the condition, the maneuver order of observer is not necessarily higher than that of the target, this conclusion is different from the previous ones. The method adopted in the paper is also effective to the general bearings-only measuring problem.