The photoelectrochemical behavior of p-type boron phosphide（BP） in acidic solution was investigated. The p-BP photocathode had a good wavelength response(E_g=2.0 eV)and was suitable in an sulfuric acid solution under irradiation. The gas evolved from this BP photocathode under irradiation was hydrogen which was produced with a current efficiency of about 100 %. The photocurrent was markedly increased by a suitable surface ruthenium treatment of the BP. A photoelectrochemical cell of the form Pt(H₂)/0.5 M H₂SO₄/p-BP was shown to have an the open-circuit photovoltage of 0.45 V, the short-circuit photocurrent density of 2.4 mA·cm^-2, and the fill factor of 25 %.