Aimed at the status quo of difficult to detect hydraulic support straightness error，a method of assessment hole axis straightness by measuring the chord length to determine the center of the cross section of hydraulic cylinder is presented. The laser was used as a reference standard for a straightness error assessment, and designed a straightness detection device that based on pneumatic measurement and laser collimation. The pneumatic measurement scheme in nozzle and curved surface baffle plate model was analyzed, analysis of the spatial location of Position Sensitive Detector (PSD) sensor was carried out and a method utilizing PSD sensor to separate error was presented. By using this device detected the straightness error, the detection process is stable and can meet the requirements.