In precision machining, it is desirable to measure the workpiece form profiles to provide feedback for control to maintain machining quality. A water beam assisted form error in-process optical measurement approach has been proposed to solve the opaque coolant problem to permit form error in-process optical measurement. To have more comprehensive understanding of the method, factorial and preliminary parameter tests were conducted. It was found that the water flow rate Qw and the height of medium hm are the two most important parameters affecting the transparent window size At. For Qw, there is a transition of flow state changing from laminar to turbulent. Increasing hm generally gives smaller At. The preliminary parameter test results also show that the conditions Qw[0.6-0.75ml/s] and hm [0.3-0.4 mm] give better stability for At.