In order to improve seismic behavior of one-shaped concrete squat shear wall with built in vertical steels in the rectangular section, two concrete squat shear wall specimens were designed and fabricated with an aspect ratio of 1.0, which enhanced by horizontal steel plates other than built-in vertical steels in the sections. Mechanical behaviors of those specimens were tested under reversed cyclic lateral loads and a constant vertical load. The test results were compared with a specimen without enhancement of horizontal steel plates, which indicates that the peak lateral loads of specimens with steel plates are improved significantly. The ultimate drift ratios of specimens with steel plates are improved by 20% and are larger than 1/100, which meets the relevant specifications. Steel plates change the failure modes of specimens, and the total energy dissipation of specimens with steel plates are improved substantially. The enhancement should be kept in a suitable range, otherwise new weak areas will be formed if the horizontal steel plates are too strong.