Using the variable of spatial instead of time, the dynamics of soil physiochemical properties and undergrowth vegetation characteristics of Populus×Euramercana ‘Neva’ and Robinia pseucdoacacia artificial mixed and the corresponding pure forests in different ages in the coastal saline-alkali area of the Yellow River Delta were addressed in this research. The results showed that:(1) Imperata cylindrica, Salsola collina, and Cirsium setosum were the dominant species in the undergrowth community of plantations. (2) With increasing plantation ages (3 a, 7 a, and 18 a), the Patrick index, Margalef index, Shannon-Wiener index, Simpson index, and Pielou index of Populus×Euramercana ‘Neva’ and Robinia pseucdoacacia artificial mixed forests and their pure forests appeared a low-high-low variation trend; 7 a artificial forests had the highest richness, uniformity, and diversity of herbs. (3) Populus×Euramercana ‘Neva’ and Robinia pseucdoacacia mixed forest had more species of understory herbs and higher importance values. As far as pure forests are concerned, the species diversity of the understory vegetation of the Robinia pseucdoacacia forest was significantly higher than that of the Populus×Euramercana ‘Neva’ forest. (4) With stand age increased, the soil pH value of mixed and the corresponding pure forests decreased gradually. The soil salinity of pure forests increased first and then decreased, and that of the mixed forests decreased gradually. The total phosphorus content of mixed and the corresponding pure forests first increased and then decreased. The total nitrogen content of the Populus×Euramercana ‘Neva’ pure and mixed forests first decreased and then increased. Soil total nitrogen content in the Robinia pseucdoacacia pure forests exhibited a continuously increasing trend. Soil pH value, salinity, total nitrogen, and total phosphorus among the three age stands were significantly different. (5) There was a significant negative correlation between the index of herbaceous diversity and soil salinity, and a significant positive correlation with soil total phosphorus.