<p>Quasi-zero stiffness (QZS) isolators in the vertical direction have been deeply studied because their working states can be easily achieved through the gravitational force of the loaded mass. However, in the horizontal direction, there is no the constant force similar to the vertical gravitation of the loaded mass to constrain the loaded mass in the working states. Therefore, a novel horizontal isolator composed of a QZS unit and a constant force unit is proposed. The two units respectively consist of oblique bars and tension springs, which can realize the constant force, constant QZS, and nonlinear QZS. The constant force unit enforces the loaded mass in the working state to obtain QZS in the horizontal direction. The parameter conditions of QZS and constant force are derived, and the effects of parameters on QZS features are analyzed. The equation of motion by considering the constant force is solved using an increment harmonic balance method integrated with the arc-length continuation algorithm. The theoretical results show that the isolator has five solutions at one frequency due to the transmissibility bending to left and then to right under the large constant force. A prototype is fabricated and tested to verify the theoretical prediction. The experimental results show that the proposed isolator has more excellent performance with lower transmissibility and wider frequency band than the corresponding linear isolator. This study provides a feasible approach to designing the horizontal QZS isolators.</p>