This paper proposes a three-step interval estimation method in the framework of optimization for discrete-time switched systems. The proposed method is realized based on observers design via reachability analysis and measurement update. In order to obtain interval estimation, a peak-to-peak index is used to improve the robustness caused by disturbances and noise. The calculation of zonotopes and boxes is used for reachability analysis. A new iterative algorithm is proposed to reduce computational complexity. For improving estimation accuracy, a method of 1-norm optimization is proposed, which can reduce the conservatism of computing intersections.