Disclosed is a chair chassis, wherein a seat bearing plate is disposed above a main tray base, for bearing a seat, a front end of a movable support member is pivotally connected to the seat bearing plate, a lower end thereof is pivotally connected to the main tray base, the movable support member is pivotable, a pair of movable supporting plates are located in front of the movable support member, upper and lower ends of each movable supporting plate are respectively pivotally connected to the seat bearing plate and the main tray base, and a pivot where the movable support member and the main tray base are connected is mounted with an elastic member for resisting the movable support member from rotating rearward. A user is subject to appropriate pressure when leaning back against the backrest, and friction between the backrest and the user's back may be reduced.