Abstract: For the popularization of Pinus densiflora var. zhangwuensis, a new afforestation tree species on the desertified and salinized-alkalized lands in Northern China, and to  evaluate the salinityalkalinity tolerance of the tree species and to better understand the tolerance mechanisms, a pot experiment with 4year old P. densiflora var. zhangwuensis and P. sylvestris var. mongolica was conducted to study their seedlings growth and physiological and biochemical indices under the effects of three types salt (NaCl, Na₂CO₃, and NaHCO₃) stresses and of alkali (NaOH) stress. Under the saltalkali stresses, the injury level of P. densiflora var. zhangwuensis was lower, and the root tolerance index was higher. The leaf catalase (CAT) activity increased significantly by 22.6 times at the most, as compared with the control; the leaf malondialdehyde (MDA) content had no significant increase; the leaf chlorophyll (Chl) content had a smaller decrement; and the leaf water content (LWC) increased slightly. P. sylvestris var. mongolica responded differently to the salt-alkali stresses. Its leaf CAT activity had less change, MDA content increased significantly, Chl content had significant decrease, and LWC decreased slightly. It was suggested that P. densiflora var. zhangwuensis had a greater salinity-alkalinity tolerance than P. sylvestris var. mongolica. The higher iron concentration in P. densiflora var. zhangwuensis needles enhanced the CAT activity and Chl content, whereas the higher concentrations of zinc and copper were associated with the stronger salinity-alkalinity tolerance.