The tracking convergent property is an important guideline for a reticle seeker. The tracking of the reticle seeker is driven by the period mean torque which is generated by the tracking signal. In this paper, the convergent property of the reticle seeker was analyzed and some significant conclusions were attained using series theory. The convergence of the tracking procession was affected by both the tracking coefficient and the phase bias Δθ between the target location and the tracking signal. The convergent conditions and the convergent step function were also obtained. These conclusions are important for the design of the convergent tracking algorithm of the reticle seeker.