Abstract: During formation fracturing with high energy gas, the successful key is to design an optimum charge that can produce a reasonable peak pressure which both,can fracture formation and can effectively protect well wall and casing from damaging and deforming. In this paper, considering factors such as the explosive gas in gas generator, killing fluid, casing, formation and so on, established a mathematical calculation model of charge versus pressure based on the first law of thermodynamics, and compiled a relevant calculation software. The maximum error between calculated and measured pressures(measured for 12 times on site) is 4. 63 MPa, the relative error is only 5. 65% comparing with the measured peak pressure. The results are satisfactory.