Abstract: In order to improve the precision of the innermost area effects in altimetry geoid computed by the Molodensky formula, deflections of the vertical were expressed as double quadratic polynomials regarding the innermost area as a rectangular one, and the formulae to calculate gravity anomaly of this area were derived after the non-singular transformation was introduced. The analysis based on the theoretical model of deflections of the vertical shows that these formulae’s errors are zero, and traditional formulae’s erros depend on the latitude and the ratio of the north-south deflection component to the west-east deflection component. A practical calculation was done based on deflections of the vetical data with a resolution of 2'*2' in the middle latitude area. The results indicate that the RMS differences between the contributions of the grid including the calculation point calculated by traditional formulae and this paper’s formulae are several centimeters. The formulae derived in this paper could provide theoretical basis for the recovery of altimetry geoid with high precision.