A method and an apparatus for controlling a TX power amplifier are provided. The method is for generating a power control curve in an inter-slot region between two continuous or adjacent time slots, which respectively have different modulation types. The method and apparatus produces a concave inter-slot ramp control curve for reducing the power of the TX power amplifier in the inter-slot region, where the power for signal transmission is transformed from a first power value K1 to a second power value K2. The method and apparatus further estimates a transient period of the concave inter-slot ramp control curve, during which modulation switching of the TX power amplifier may occur.