Aiming at effective external trigger of high speed linear image acquisition system in practical application, a trigger delay checkout method was presented, for setting the trigger correctly and getting dynamic target image reliably. A double screens device is used to obtained velocity of dynamic target, the second screen provide trigger signal to linear image acquisition system, the time that system received trigger signal is computed according to target velocity and screens parameters; the time that system start to capture is computed by analysis of sequence linear image captured by system, then trigger delay is checked out. For a specific type linear image acquisition system with 64 000 line per second (lps), trigger delay is measured, the test results analysis show that the linear image acquisition system has external trigger delay, the average delay time is 152 μs, the delay variance is 3.3 μs, and the delay error is less than 1 line scan cycle. The factors leading to measurement errors and action principle are analysed, and a common method to design measurement system parameters are presented by two external trigger delay measurement precision analysis examples of line scan camera with 10 000 lps and 50 000 lps line scan frequence.