A single broadband microstrip patch antenna (MPA) under small thickness is proposed by using multi-resonant modes. Initially, the linear slots in the same direction is cut on the rectangular radiator with four shorting pins. It aims to make use of TM30, TM12, and TM10 modes of the MPA. After that, the wideband split-ring resonator (SRR) network is applied to generate two additional resonant modes. Finally, the simulated results has formed that the relative working bandwidth of our selected antenna is obviously broadened to about 25.9% with a small thickness advantage of about 0.042 lambda-{0}. Moreover, it holds the almost constant beam for its normal radiation patterns.