In this paper, we describe a method for controlling wearable walking support system which is referred to as Wearable Walking Helper. In this method, the support moment for the user's knee joint is calculated with an approximated human model and a part of it is supported by the prismatic actuator of Wearable Walking Helper. In the conventional control algorithm, we only considered to support the user to walk on the level ground and assumed that horizontal components of Ground Reaction Force (GRF) could be neglected. In this paper, we especially focus on the support of walking on slope, and estimate slope angle based on sensor signals from accelerometer and force sensors. By using the estimated slope angle, horizontal components of GRF are calculated and we apply it to the calculation method using the human model. The proposed method is applied to Wearable Walking Helper and the experimental results illustrate the validity of it.