A resistance apparatus for a waist-twisting exerciser wherein the resistance apparatus includes a spring and an adjuster with a pulley block. The adjuster is mounted on a swing element while the spring is fixed at a proper position of the swing element such that the spring is correctly engaged into the pulley block of the adjuster. In applying force to the swing element in motion, the spring may be pulled to create an effective exercise resistance force for increasing the exercise effect. Moreover, the position of the adjuster is adjustable for changing the tension of the spring.