This paper introduces a recently proposed effective control strategy for the propagation constants of endfire leaky-wave antenna (LWA) with two kinds of loading stubs within and at two sides of the coplanar stripline to adjust the phase and attenuation constants separately. The inner stubs can decide the phase constant value of the proposed LWA, while the outer ones are employed to control the attenuation constant in a large range with weak influence on the phase constant value. The designed LWA can also maintain the phase value and slope in the operation range under different attenuation constants. In this way, a specific radiation pattern can be acquired easily in a frequency range by tailoring the current amplitude along the antenna with matched phase. An endfire LWA with low sidelobe pattern is designed and fabricated to verify the effectiveness of provided method in controlling propagation constants separately. The well-matched results between simulation and measurement confirm the predicted radiation properties of the designed LWA, and the proposed strategy can modify the attenuation constants effectively with almost fixed phase results in the endfire operation range.