Stand structure is an important feature reflecting the status of forests. Here, four typical secondary forests in the Greater Khingan Mountains are examined. Five random plots were selected from each forest, and their tree size distributions and stand spatial structures were compared. The unary distributions of the diameters at breast height (DBH) and tree heights showed consistency, which indicated there was an over-abundance of small-diameter trees in these forests. The joint probability distribution showed that the number of small-diameter trees was significantly larger than that of large-diameter trees. Furthermore, according to the unary distribution of spatial structure characteristics, the stands mostly exhibited a clustered structure, and the competition in the stands was intense, which is not conducive to growth. One-way covariance analysis was used to test the similarities and differences in the spatial parameters of the four stands. There were no significant differences between them in terms of dominance or the uniform angle index. Only mingling was significantly different, where the mixed forest had a significant advantage (p = 0.05). The structure of these secondary stands is poor, so the formation of a stable forest ecosystem by natural succession is hampered.