This research develops a rapid z-position measurement based on the light scattered from the passing particles under a field of light with chromatic aberration. Two band pass filters and two avalanche photodiode (APD) are used for rapid detecting the scattered light from the passing objected. The intensity ratio of two selected band 430 nm-470 nm (blue light) and 630 nm-670 nm (red light) from the scattered light becomes a significant indexes for the depth information of the detecting objects. The throughput of the developed system is capable to for detecting fast moving objects of upto 1000 particles/s which is about 10 folds compared to the spectrometer system. Human erythrocytes is also successfully detected without fluorescence labeling at a high flow velocity of 2.8 mm/s. With this approach, quantitative measurement for the depth position of rapid moving objects inside a sealed microfluidic channel can be achieved in a simple and low cost way.