1. A fitness ball chair, comprising: a base, being an elastic structure formed by being spirally encircled and having an opening formed in the middle of the base; a fitness ball, being an inflatable ball movably disposed in the base, and having a portion disposed in the opening to prevent the fitness ball from falling out; a support tube, having two ends which are a first end and a second end, and the second end being coupled to the base; a pad, installed at the first end, for supporting a user's trunk, and providing a protection effect to the user by the softness of the fitness ball and the elasticity of the base during the use of the fitness ball.