The invention discloses a triangular cycle and Rankine cycle combined waste heat recovery system and a method thereof. The waste heat recovery system comprises working medium pumps, evaporators, expansion machines and condensers. In a triangular cycle, a working medium enters the high-temperature cycle evaporator, absorbs heat, always maintains a liquid state, and then, enters the expansion machine to act, and waste steam after acting preheats a low-temperature working medium in a Rankine cycle in the high-temperature cycle condenser; and in the Rankine cycle, the working medium enters the high-temperature cycle condenser, is in heat exchange with waste steam in the triangular cycle, then, enters the low-temperature cycle evaporator, is in counterflow heat exchange with low-temperature tail gas which is already in heat exchange with the high-temperature working medium in the high-temperature cycle, and then, enters the low-temperature cycle expansion machine to act. According to the triangular cycle and Rankine cycle combined waste heat recovery system, the working medium isothermal heat absorption process in the traditional Rankine cycle in the high-temperature cycle evaporator can be eliminated in the triangular cycle, the temperature matching problem between a heat source and the working mediums is solved, energy loss of the system is reduced, and efficient recycling of waste heat of automobile tail gas is facilitated.