Deep-hole drilling (depth-diameter ratio > 10) accounts for about 40% of hole processing. As an important hole processing technology, the gun drill has a complex structure and weak relative stiffness, which makes it difficult to avoid the straightness deviation of the hole. In addition, the current theoretical model of straightness deviation is inconsistent with the actual working condition of long and deep hole gun drilling, so the prediction of straightness error is still a difficulty in deep hole machining. Based on the analysis of influencing factors of straightness deviation of gun drilling, this paper establishes the theoretical model of the straightness error of the middle four supports by using the Euler-Bernoulli beam theory and reveals the relationship between the bit deflection and the deviation of the hole axis. In this paper, the influence of support clearance, support spacing, support stiffness, axial force, and drill pipe stiffness on straightness error is analyzed through simulation and experiments. The method to reduce straightness deviation by controlling support clearance is proposed and verified by experiments.