The complete distribution of high‐energy photon primary absorbed dose along the beam path within a semi‐infinite or finite water phantom can be expressed by one functional equation. The derivation is based on a technique of convoluting a primary collision kerma distribution function with a pair of forward and backward spread dose‐distribution functions. In particular, the characteristic patterns of primary dose in the regions near the beam entrance and exit surfaces can be obtained. The calculated ratio of primary dose to primary collision kerma in the electron‐equilibrium region in water for60Co radiation is 1.0060 and that for 25‐MV radiation is 1.0444.