Horizontal wells are frequently used in gas reservoirs to enhance the production performance by increasing the contact surface of the well with the formation. Although some methods have been presented to evaluate vertical well productivity in tight gas reservoirs, the detailed information regarding horizontal wells has not yet been deeply addressed. This study uses a modified backpressure test method based on the isochronal test to determine the correct value of the productivity of horizontal wells in a pressure-dependent viscosity anisotropic tight gas formation. To use the isochronal test calculations in the modified backpressure test, the process conversion-flowing pressure correction approach was used to convert the modified backpressure test process into the isochronal test. The comparison between productivity values before and after correction with the simulation results validates that how this method can successfully estimate the productivity in horizontal wells. We perform a sensitivity analysis on the error between the pseudo-pressure before and after correction and show its variation with porosity, permeability, skin, formation thickness, and temperature. This can be helpful to understand the importance of correcting pseudo-pressure under different conditions. Results validate that the proposed method of productivity analysis is needed for horizontal wells and should be applied especially in low permeable and thin (tight gas) formations to acquire the precise value of productivity.