Large length-diameter ratio peg-in-hole assembly is a hard and significant issue in modern industry. The prediction and control of the contact force/torque play a crucial part in flexible assembly. They ensure that the contact force/torque data are within limits in assembly process, which can prevent jam and protect devices. In this paper, we propose a novel contact force/torque prediction and analysis model to solve the large length-diameter ratio peg-in-hole assembly problem in which the contact states are difficult to be obtained. Firstly, we establish a new force/torque prediction model with measured data to obtain the precision actual contact force/torque which is critical for assembly control. Then, a new contact analysis model for large length-diameter ratio peg-in-hole assembly is built to estimate the assembly contact states. At last, based on the proposed contact force/torque prediction and analysis model, we design a robot pose adjustment strategy for large length-diameter ratio peg-in-hole assembly. Experiment results demonstrate that the proposed model can meet the demands of large length-diameter ratio peg-in-hole assembly. The predicted error rates of force/torque are lower than 1% and the mean force/torque in assembly process are lower than 5 N / 0.5 N·m by our model.