Abstract: In all kinds of experiments using air track with accessories and spare parts，whether direct or indirect measurement，velocity is a basic physical quantity to be determined and the accurate measurement of velocity is of great significance. Universal counter MUJ-6B is often used to record the blackout time as different type of light barrier passes through the optical gate，while the vernier caliper is used to measure the blackout distance，then the value of velocity is obtained. From the viewpoint of the principle of photoelectric timing，this article analyzes the reason that there is no systematic error with U-shape light barrier using timing mode 2，while there is great systematic error with strip light barrier using timing mode 1. Moreover，the“swallowed”width of strip light barrier by photoelectric gate is deduced by using the theoretical formula of harmonic vibration model which is realized by two springs and a slider moving on the air track. Hence，the modified formula of velocity is obtained，which is tested and verified by the experiment of momentum conservation on the air track. Therefore，the proposed velocity correction method is tested to be reliable from another aspect.

Key words: strip light barrier, U-shape light barrier, velocity correction, photoelectric timing