Preheat the oven to 350 degrees F (175 degrees C).
Spread crushed strawberries on the bottom of a 9x13-inch baking dish; sprinkle gelatin powder over strawberries, then top with mini marshmallows.
Combine cake mix, water, eggs, and oil in a large bowl; beat with an electric mixer on low speed until moistened. Increase speed to medium and beat for 2 minutes. Pour batter on top of marshmallows.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 40 to 50 minutes.
Cool cake in the pan for 15 minutes. Run a knife around edges, then turn out the cake onto a serving plate, strawberry side up. Store cake in the refrigerator.
