The invention discloses a grain oil acid value automatic measuring instruments which includes mainframe, electrode bar and titration cell, single-purpose agitator, the electrode bar is connected with the mainframe via cable, it is characterized in that: said agitator possesses oscillating slide bar 5, said electrode bar 1 and titration cell 2 are separately set on rack 4 and 3 of said single-purpose agitator, the rack 4 is mounted on said oscillating slide bar 5, lower end of electrode bar 1 is placed in liquid of titration cell 2; said electrode bar 1 is integrated composite electrode bar which possesses two working electrodes and two indicating electrodes; said mainframe possesses titration terminal judging device and interval titration device, power supplying circuit is constant-current source circuit. The detector can avoid series of problems induced by using marking liquid in traditional titration detection and analysis, it does not need marking liquid, and design is cheap, observation is eye-catching, usage is convenient, operation is automatic, it fits for large scale production and usage widely.