To resolve the long-range displacement and displacement field measurement by using optical method in engineering environment, the influence of image blurring and image position's change on white light digital speckle correlation method are investigated in the case of slant optical axis, and theoretical formula of the error was given. To weaken the influence of image position's change, the reference measurement technology was introduced and a new high-resolution method of measuring in-plane displacement measurement in the case of slant optical axis was given. The static flexibility measurement of object at distance of 2～50 m is accomplished with the maximal relative error of 1.0%, the technology has a better accuracy both in laboratory environment and engineering environment. It demands no circumstance requirements in common optical axis and is especially effective in flexibility measurement of bridge and cloverleaf junction. It also fits for dynamic displacement measurement when a high-speed image card was used. This technology expands digital speckle correlation method's application field.