In　this　paper,　the　chaotic　behavior　of　a　planar　restricted　four-body　problem　with　an　equilateral　triangle　configuration　is　analytically　studied.　Firstly,　according　to　the　perturbation　method　of　Melnikov,　the　planar　restricted　four-body　problem　is　regarded　as　a　perturbation　of　the　two-body　model.　Then,　we　show　that　the　Melnikov　integral　function　has　a　simple　zero,　arriving　at　the　existence　of　transversal　homoclinic　orbits.　Afterwards,　since　the　standard　Smale-Birkhoff　homoclinic　theorem　cannot　be　directly　applied　to　the　case　of　a　degenerate　saddle,　we　alternatively　construct　an　invertible　map　f　and　check　that　f　is　a　Smale　horseshoe　map,　showing　that　our　restricted　four-body　problem　possesses　chaotic　behavior　of　the　Smale　horseshoe　type.