Antenna sensors designed on the basis of the antenna principle have both sensing and signal transmission functions, which can effectively solve the wiring complexity problem and reduce installation cost from traditional sensors in structural monitoring. In this study, we designed a microstrip patch antenna sensor (MPAS) combined with theoretical deduction and simulation analysis. The radiation patch of the antenna was slotted to resonate within the target frequency band, achieving a reflection coefficient (S11) of less than −10 dB and excellent circular polarization, ensuring superior antenna performances. A strain acquisition instrument was employed to acquire the bidirectional strains (εx, εy) of a cantilever beam end surface-attached MPAS, while the input impedance (Z) and S11 of the MPAS were measured using a vector network analyzer, whereby the bidirectional strain sensitivity (GFx, GFy) and linear stability of the MPAS were obtained by fitting the relationship between εx, εy and Zmin or S11. The results showed that the MPAS can sense well εx and εy. Fitting the normalized S11 and εx, εy yields high GFx and GFy values of 141 and −140 ppm/με with superior goodness-of-fit values of 0.978 and 0.979, respectively, which indicate a sensitivity much higher than those of conventional strain sensors.