Different from the ordinary cockpit displays，the background of helicopter head-up display is the real outside scene and thus the readability of the displayed picture is easily disturbed by the outside environment. A method was proposed based on Perceived Just-Noticeable Difference (PJND) theory for evaluating the readability of helicopter HUD in complex visual environment.The relationship between the satisfactory PJND value and the environment factors (luminance and chrominance) was studied through experiments. On this basis，a luminance control curve satisfactory for the readability of HUD was designed. Experiment results showed that:1) The PJND value can synthetically reflect the effect of luminance and chrominance on the readability;and 2) The luminance control curve designed based on PJND value can satisfy the requirement on readability of the helicopter HUD.