The study of the origin of asymmetries in mirror $\beta$ decay is extremely important to understand the fundamental nuclear force and the nuclear structure. The experiment was performed at the National Laboratory of Heavy Ion Research Facility in Lanzhou (HIRFL) to measure the $\beta$-delayed $\gamma$ rays of ${}^{26}$P by silicon array and Clover-type high-purity Germanium (HPGe) detectors. Combining with results from the $\beta$ decay of ${}^{26}$P and its mirror nucleus ${}^{26}$Na, the mirror asymmetry parameter $\delta$ ( ≡$f{t}^{+}$/$f{t}^{-}$− 1) was determined to be 46(13)% for the transition feeding the first excited state in the daughter nucleus. Our independent results support the conclusion that the large mirror asymmetry is close to the proton halo structure in ${}^{26}$P.