Continental tight oil reservoir is developed in Block 4+5 of Block Z in central Ordos basin. The evaluation of effective reservoir lower limit parameters of tight reservoir has important guiding significance for dessert prediction and reservoir waterflooding development. In this paper, the interpretation method of effective porosity of tight oil reservoir is established by using a large number of lithology, physical property data, logging data and mercury injection data of Chang 4+5 in this area. the lower limit standards of lithology, physical property, electrical property and oil-bearing property suitable for the evaluation of Chang 4+5 reservoir in Z block are determined. The results show that the main rock type in the study area is feldspathic sandstone, followed by lithic feldspathic sandstone. The results of thin section identification show that the main pore types include intergranular pores, intergranular solution pores, cuttings solution pores and feldspar solution pores. The porosity of the Chang 4+5 reservoir group is concentrated in 8%-14%, while the permeability is concentrated in the range of 0.04×10-3 μm2 ~1×10-3 μm2. The results of core observation, experimental analysis and comprehensive interpretation of logging parameters show that the lithologic lower limit of tight sandstone reservoir in this area is fine sandstone, corresponding to the lower limit of oil-bearing grade; the lower limits of physical and electrical parameters are: porosity 9%, permeability 0.2×10-3 μm2, oil saturation 40%, resistivity 15 Ω?m, acoustic time difference 226.4μs/m.In addition, there is a certain positive correlation between sandstone thickness and effective thickness, and there is also a certain positive correlation between effective thickness and porosity. When the reservoir physical property is better and the sand layer thickness is larger, the reservoir thickness is larger; on the contrary, when the reservoir physical property is poor and the sand layer thickness is small, poor oil layers are mainly developed.