Porosity is an important parameter of reservoir rock，which is mainly used to evaluate reservoir physical properties and to calculate original reserves of petroleum. Pore structure characterization of tight oil reservoir is mainly on micronano scale. The tight oil reservoir porosity is generally less than10%. It is difficult to measure tight oil reservoir porosity accurately and quickly using conventional methods. Based on Boyle’s law and pressure differentiation function of porosity measuring，parameters affecting the accuracy of porosity measurement were analyzed. Also，experimental instrument parameters were optimized. Experimental apparatus for porosity determination，which is suitable for the tight oil reservoir，was designed. The porosity of a core sample is 2.55% using the experimental apparatus，while the core porosity is 2.40% using mercury intrusion method. The two results agrees. In order to verify the accuracy of the experimental apparatus，the porosity of an artificial core which is 25.16% was measured using the optimized experimental apparatus. The result is 25.56%.Therefore，porosity of the tight samples can be quickly and accurately determined by the optimized experimental apparatus.