A new isotope $^{203}$Ac was produced in the reaction $^{40}$Ca + $^{169}$Tm at beam energy of 226 MeV. One correlated α-decay chain was attributed to $^{203}$Ac. The α-particle energy and half-life were determined to be 8217(16) keV and ${56}_{-26}^{+269}$ μs, respectively. The reduced α-decay width (${\delta }^2$) of $^{203}$Ac was deduced to be ${1096}_{-906}^{+921}$ keV, which indicates a favored α decay. The systematics of ${\delta }^2$ and single-particle energy levels indicate that the new state probably arises from the intruder $s_{1/2}$ configuration, which shows an onset of deformation in Ac isotopes. The proton separation energy $S_p$ of $^{203}$Ac was extracted as $-1214\left(22\right)$ keV and the systematics of $S_p$ for odd-A isotopes beyond proton drip line were discussed.