To estimate UAV (unmanned aerial vehicle) pose relative to airfield runway, a posterior parameter estimation method using the runway image and onboard pose sensors is presented. Firstly, pose parameters of UAV are measured by onboard sensors such as altimeter, GPS (global positioning system) and INS (inertial navigation system). Supposing the prior probability distribution of each parameter's measurement error is given and all errors are independent, the measured values from sensors can be considered as a sample of real ones. At the same time of sampling, an image of the runway is acquired from onboard calibrated camera, and the two edge lines of the runway are assumed to be extracted exactly from the image. Subject to the constraint of the perspective view of two parallel lines, the real parameters must locate on a constraint surface in the parameter space, and thereby the posterior mean estimation of the parameters can be obtained. Simulation results show that the method is effective.