A combined method is proposed to determine the water entry acceleration at a low impact velocity through image processing.The procedure includes:(1) a sequence of images for water impact are recorded by a high speed camera,(2) the sub-pixel image processing method is employed to calculate the displacement with an accuracy on the “sub-pixel” level,(3) the acceleration of the object is acquired by differentiating the displacement twice and with results being further filtered by a carefully designed low-pass Butterworth filter.A theoretically based analysis is conducted for designing the parameters of the low-pass filters.It is shown that the water entry can be regarded as a procedure with a slowly changing velocity.The method is validated with the standard sinusoidal motion and the water entry of a sphere.This approach could be considered as an auxiliary method during the early-stage study of the water entry,and it could be further applied to some complicated circumstances,like the water entry of spinning spheres.