In the framework of the 3D bond fluctuation model the phase transition of asymmetric polymer mixtures is studied in the semi-grand-canonical ensemble by a Monte Carlo simulation. The asymmetry is realized by different chain lengths (N-B/N-A = 2 or 3) of the two types of polymers. Monomers of the same kind interact via an attractive square well potential extended over the first peak of the pair correlation function, whereas monomers of different types repel each other. We determine the equation of state and discuss the dependence of the critical temperature and density on chain length and asymmetry. The results are related to predictions of the Flory-Huggins theory via the intermolecular pair correlation function of the macromolecular fluid.