Cylindrical charge is a main form in most application of explosives. By employing numerical calculation and an indirect mapping method, the relation between peak pressures from underwater explosion of cylindrical and spherical charges is investigated, and further a model to predict full-field peak pressures by the former one is developed and then verified by experiments. Besides, a method to intuitively sketch the corresponding orientational performance is proposed and discussed. The results indicate the following: when R/R-0 ranges from 12 to 100, the map- ping relation is nearly dominated by the amplitude similarity constant K-p, and an approximate shifted sine relation between K-p/K-0 and action angle exists; in cases that L/d is over 1.5, the prediction model shows a good agreement with experimental data, with most errors below 10% and it is also universally applicable for different explosives; the directivity of peak pressure distribution in underwater explosion is continuously improved along the increasing L/d but no significant influence appears after L/d becomes over 5.