Current publication is devoted to the construction of modification of well-known Beddington - Free - Lawton model of parasite - host system dynamics and its analysis. In basic model and in modified model there is one and the same assumption that under the determined values of sizes of interacting populations number of attacked hosts is determined by Poisson' law. In modified model it was took into account that hosts, which were attacked two or more times by parasites, die, and it doesn't lead to the increase of parasite population size. For modified model population dynamic regimes and structure of the space of model parameters are analyzed.