Based on the description of drilling cores, observation of thin sections, microscopic and Raman analysis of fluid inclusions in deep natural gas reservoirs in Minfeng sag in dongying depression, the types of acid dissolution in reservoirs were described in this paper, done same to the characteristics of fluid inclusions. The observations show it is an important mechanism of second pore forming in deep natural gas reservoirs by acid dissolution in Minfeng sag. The dissolutions of carbonates, feldspars and rock bits are obviously, however, the quartz mainly dissolved at the edge grains. The compositions of fluid inclusions in reservoirs were analyzed by Raman spectroscopy. Experimental results indicate that the porosity and filter rate of reservoirs rocks increased with the content carbon dioxide in inclusions, but the content of carbonates in rocks decreased accretion of the content of carbon dioxide in fluid inclusions. The Raman spectra of gas hydrocarbon-bearing inclusions show that hydrocarbon co-exist with carbon dioxide. It proved that the acid fluids occurred during the accumulation of natural gas. So, we know that the second pore in reservoirs increased due to the dissolution of cementation caused by carbon dioxide, such as dissolution of carbonates. This process accelerated the accumulation of natural gas. The undersaturation hydrocarbon was found in fluid inclusions, which give direct evidence for the thermal cracking origin natural gas. This study give reliable evidence for acid dissolution of deep reservoirs in Minfeng sag, and the results imply that it should be cared the zone of second pore caused by acid dissolution in future deep exploration.