The calculation of oil and gas reserves by the probability method represents the trend in the oil industry. To study the reserves per unit volume of Ordovician fracture-cavity carbonate oil reservoir in Tahe Oilfield，we fitted the reserves per unit volume in cave，vuggy and fractured reservoirs with the probability method and established a probability distribution model. The research demonstrates that the reserves per unit volume of fracture-cavity reservoirs in Tahe Oilfield conform to the Beta distribution model. To be specific，the reserves per unit volume in cave reservoirs are the highest，and the Pmean value calculated by the probability method reaches 10.7×104 t（/ km2·m）. The reserves per unit volume in vuggy reservoirs are 0.61×104-2.2×104 t（/ km2·m），while those in fractured reservoirs are 0.08×104-0.19×104 t（/ km2·m）. The geological distribution of reserves per unit volume in fracture-cavity reservoirs is clarified. It is pointed out that the scale of fracture-cavity reservoirs and crude oil density greatly influence the reserves per unit volume，revealing a positive correlation. The larger reservoir scale and the higher crude oil density indicate the greater reserves per unit volume. Under the same geological conditions，the reserves per unit volume based on the sample values and the probability method are similar，showing a slight positive correlation. As the exploration progresses，the probability method is more accurate for calculating the reserves of oil reservoirs.