Abstract: Objective To design a speed-constant rectal lavage apparatus for assessing the rectal and anal function in liquid stool continence and the degree of fecal incontinence. Method The apparatus was assembled by four common rectal lavage apparatuses, each with a container and a drainage tube. Results The device could output a pulsed and speed-constant water stream with high pressure, capable of overcoming any rectal resistance change. The test for liquid continence in 12 patients with incontinence (n=12) and 12 normal control subjects (n=12) found that the retained liquid volume was significantly lower in the incontinence group than in the control group (P<0.01). Conclusion Fecal incontinence could be evaluated with the speed-constant rectal lavage apparatus, which yields objective and quantitative results.