A beverage mixing device includes a primary container, a secondary container mounted in the primary container, a diaphragm removably mounted on the secondary container to seal the lower end of the secondary container, a sliding member movably mounted in the secondary container and having a lower end provided with a cutting blade that is movable to penetrate the diaphragm to connect the secondary container to the primary container and an upper end provided with a helical driven portion, and a top cap removably mounted on the primary container and provided with a helical drive portion abutting the helical driven portion of the sliding member to push the sliding member toward the diaphragm by rotation of the top cap relative to the sliding member.