The multiplicity dependence of heavy flavour production in pp-collisions at LHC energies is studied in the framework of string fusion approach. We applied a Monte Carlo model [1, 2], with the string configurations simulating event-by-event. We assumed that the heavy flavour yield is proportional to the number of initial strings, whereas a total charged particle multiplicity is influenced by the string fusion process and can be obtained according to the standard string fusion prescriptions. We show that under this assumption the faster-than-linear growth of the open charm production, observed in experiment [3], can be related to the reduction of the total multiplicity due to string overlapping and fusion. The influence of this effect on forward-backward correlations involving heavy flavours is also discussed.