Horizontal well technology is still applied widely in the world. Some researchers and oilfield operators of horizontal wells are still using improper productivity predicting equations and critical production rate calculations. Therefore, this paper analyzes some productivity forecast equations and critical coning rate calculation procedures for horizontal wells focusing on Joshi's equation of predicting productivity and Chaperon's equation of calculating critical coning rate used in reservoir engineering. This paper suggests that effective permeability should be used in calculating horizontal well productivity and points out an error existing in Joshi's equation. It also evaluates and introduces several equations for critical production rate. Furthermore, this paper derives two new equations for predicting horizontal well productivity and a general predictive equation for horizontal well critical coning rate, applying the Mirror Effect Theory. Field applications show that the relative error between generally accepted methods of prediction are less than 17% and the new calculation method is more accurate than that of Joshi’s equation. Moreover, using actual oilfield examples, the modified Chaperon's equation was verified.