Mixing device (1) and fluid mixing method, by means of successive transfers between syringes. The mixing method comprises placing a pair of syringes (2, 3) in the mixing device (1), adjusting a variable fixing element adaptable to the pair of syringes (2, 3) and selecting and running a mixing programme, being the mixing force, the speed and the range and the number of transfers adjustable. The mixing device comprises a mobile carriage (7) longitudinally movable and fixing elements (8, 9) with an adjustable distance between them. The invention allows for the mixing or emulsifying of blood fluids with different viscosity, particularly a protein gel and a platelet-rich plasma, for the preparation of dermatological formulations, in a versatile, hygienic and effective manner.