An instrument to measure the retinal potential visual acuity (PVA) of the patients, suffering from cataract, is studied. This instrument uses a transparent eye chart. With special optical system, the beam from the exit pupil of the instrument is thin enough to pass through the aperture of the opacity lens of eyes with cataract and forms image on the retina. The move of the eye chart makes it possible to measure the PVA of the patients with diopter -7～+12 m-1. The design of the configuration of the instrument and some theoretical calculation are finished, along with experiments and tests. The experiment shows that the diameter of the spot at the exit pupil is 0.094 mm, and the vision measured with potential vision meter (PVM) accords with that measured with traditional method. The PVA of the patients can be accurately measured with the instrument before the operation, which is important to both the doctors and the patients.