An auto-positioning hydraulic pneumatic fixture contains a base, at least one locking device, multiple pistons, multiple positioning modules, and multiple platforms. The base includes at least one installation zone for fixing at least one locking device. A respective one locking device includes a fixing module and a clamping module. The fixing module has a body, a connection segment, and an affix portion. The clamping module includes a positioning bolt and a pair of clamp units, and each clamp unit has a clamping face. The affix portion has a defining groove and a linear slot. The body has at least one tilted rail, and a respective one positioning module has a mounting unit and at least one locating unit. The mounting unit has two oblique faces and multiple fringes, and s respective one platform is configured to fix the respective one workpiece.