Unreinforced masonry walls are extensively used in the petrochemical industry and they are one of the most vulnerable components to blast loads. To investigate the failure modes and improve the blast resistances of masonry walls, four full-scale field tests were conducted using unreinforced and spray-on polyurea-reinforced masonry walls subjected to gas explosions. The results suggested that the primary damage of the unreinforced masonry wall was flexural deformation and the wall collapsed at the latter stage of gas explosion. The presence of polyurea coatings could effectively improve the anti-explosion abilities of masonry walls, prevent wall collapses, and retain the flying fragments, which would reduce the casualties and economic losses caused by petrochemical explosion accidents. The bond between the polymer and masonry wall was critical, and premature debonding resulted in a failure of the coating to exert the maximum energy absorption effect. A numerical model for masonry walls was developed in ANSYS/LS-Dyna and validated with the test data. Parametric studies were conducted to explore the influences of the polyurea-coating thickness and spray pattern on the performances of masonry walls. The polyurea-coating thickness and spray pattern affected the resistance capacities of masonry walls significantly.