A support brace has a conjoining component, an extending component, a guiding set, an adjusting set, and an operating set. The guiding set has a guiding pole mounted to the conjoining component. The adjusting set is mounted to the extending component and has an adjusting sheet with a through hole located around and engaged with the guiding pole. The operating set has a pressing pole able to press the adjusting sheet and to disengage the through hole of the adjusting sheet and the guiding pole, and make the extending component to which the adjusting set is mounted moveable relative to the conjoining component along a moving direction. The support brace is longitudinally adjustable, is suitable for different limb lengths, and is able to reduce cost of casting orthopedic cast and purchasing appliances of orthotics of different sizes.