A broadband high-gain multilayer transmitarray operating in the X-band is proposed in this work. Unlike most transmitarrays achieving phase compensation by changing their element sizes, the length of striplines of the proposed transmitarray is adjusted to control a wider range of phase responses of identical array elements. By reducing the mutual coupling between adjacent units using surrounded metallic vias, the performance of the proposed transmitarray cell units can be improved significantly. In addition, aperture-coupled bowtie slots with stacked patches are adopted for wideband matching and high gain. By exploiting the true stripline phase compensation and aperture coupled technique with mutual coupling improvement, the proposed transmitarray with a flat gain response is implemented. The designed transmitarray accomplishes an enhanced matching bandwidth covering 8.0-12.0 GHz while adopting the decoupling technique between adjacent unit cells. A square transmitarray of $10\times10$ cells forming an aperture area of $25{\lambda }_{o}^{\mathbf {2}}$ at 10 GHz is fabricated and measured. The measured realized gain is 22.05 dBi, achieving 50% aperture efficiency at 10 GHz. The 1- and 3-dB gain bandwidths are 17.6% (9.35-11.15 GHz) and 19.5% (9.25-11.25 GHz), respectively. The flat gain characteristic with high aperture efficiency candidates the proposed design for radar and satellite applications.