Microalgae are an effective soil biostimulant. However, pathways for the enhancement of plant growth are still unclear. In this study, the effects of Chlorella vulgaris (C. vulgaris) on wheat growth promotion and its direct and indirect mechanisms were investigated under hydroponic experiment condition in pots in a constant temperature indoor laboratory. Living C. vulgaris showed significant promoting effect on wheat growth in terms of root length (52.41%), shoot length (44.44%) and dry weight (13.86%). Besides the function of supplying inorganic nutrient, the organic molecules in the culture supernatant and cell extract of C. vulgaris promoted wheat growth directly through interaction with the plant roots. The culture supernatant fraction increased root length, shoot length and dry weight of wheat by 27.59%. 11.84%, 16.53%, respectively. The cell extract fraction had a larger effect with the increase in root length, shoot length and dry weight by 33.10%, 20.86% and 27.10%, respectively. Changes in the bacterial community in the rhizosphere under co-culturing of bacteria and microalgae was also investigated to determine indirect mechanisms on plant growth promotion. The results showed living C. vulgaris and rhizosphere bacteria had a synergistic interaction. Compared with initial rhizosphere bacterial community at genus level, the number of beneficial rhizosphere bacteria such as Sphingobacterium, Comamonas, Acetobacter and Mucilaginibacter significantly increased when co-cultured with the supernatant of C. vulgaris. In conclusion, considering the presence of bacteria in the soil environment, it is important to maintain the activity of microalgal cells to release extracellular polymer substances sustainably to promote plant growth.Graphical Abstract: