A microscopic approach to the investigation of the behaviour of a symmetrical binary fluid mixture in the vicinity of the vapour-liquid critical point is proposed. It is shown that the problem can be reduced to the calculation of the partition function of a three-dimensional Ising model in an external field. For a square-well symmetrical binary mixture we calculate the parameters of the critical point (critical temperature and critical density) as functions of the microscopic parameters: the parameter r measuring the relative strength of interactions between the particles of dissimilar and similar species and the parameter measuring the width of the potential well. The results obtained agree well with the ones from computer simulations.