Based on the view of geometry, the new method with the product of unit reserve factor, cross sectional area and length of transverse distribution of reservoir was developed from the traditional volumetric method with the product of unit reserve factor, oil-bearing area and thickness of reservoir, the new equation for calculating geologic reserves of stratigraphic unconformity screened reservoir was discussed, and the prediction formula for calculating the thickness of reservoir in horizontal well was analyzed according to the characteristic of reservoir. The results showed that the new method improved was applicable to the calculation of geologic reserves of stratigraphic unconformity screened reservoir and the prediction of the thickness of reservoir in horizontal well.