The invention provides a positive pressure protective helmet comprising defending headgear, head fixation apparatus, air-supply arrangement and air-supply casing, at the foreside of defending headgear mounted antifog windows, at the back mounted ventilator, at the neck mounted fixation flex, at the inner part mounted head fixation apparatus which is mounted air duct that connected air-supply arrangement and the air-supply arrangement is located in the air-supply casing. Said protective helmet supplies filtered air by toxicant filtering and vent portable means, keeping positive pressure when breath out or breath in. And this protective helmet has water vapour permeability and can obstruct poison solution, gas, and particulate outside clothing and low cost respecting one time insulation garment.