We investigated NH+4/NO-3 absorption characteristics of Betula platyphylla seedlings using solution cultivation. The results showed that the seedlings preferred nitrate over ammonium in their nutrient regime. Absorption capacity of NH+4N, NO-3N and total N increased with the increase of nitrogen application or light intensity. Most of nitrate was reduced in roots; nitrate reductase activity in roots and leaves responded to increased nitrate availability or increased light intensity. Medium pH value decreased in the low Napplication solution of high NH+4/NO-3 ratio, and increased with low NH+4/NO-3 ratio; medium pH value decreased more markedly in the high Napplication solution of high NH+4/NO-3 ratio, but did not increase sharply in NO-3 solution; medium pH value of all the seedlings decreased with increased light intensity.