The beam conversional properties of Gaussian beams through a telescope-lens system were deduced based on parameter

q

and

ABCD

law. The properties of the system were simulated and calculated

and the results were testified by the focusing experiment through a He-Ne laser with plate-cocave cavity. The focusing beam radius' ratio between the telescope-lens compound system and the single lens increases via length (70~400 mm) increasing

and the maximum ratio is 30

meanwhile the ratio between the telescope-lens compound system and the single lens decreases via length (70~400 mm) shorting

but the maximum difference of the focusing beam radius’ is 0.043 μm. Experiment results suggest that choosing the telescope-lens compound system as focusing optical elements and setting the length in the shortest or zero will get a upstanding focusing purpose.