Mycorrhizal symbiosis has been evolved to be ubiquitous in natural and agricultural ecosystems. Mycorrhiza formation helps host plants acquire more nutrients and water, thereby improving host plant resistance to abiotic and biotic stresses. Molecular crosstalk begins between symbiotic partners before the establishment of mycorrhizal symbiosis. Signaling molecules and small secreted proteins are then released from the two symbionts. Signaling molecules released from the fungi include Myc factors, indole 3-acetic acid, and hypaphorine, etc. Meanwhile, they secrete some carbohydrate active enzymes (e.g., proteases and lipases), and proteins with conserved LysM and CFEM motifs. These secreted signaling molecules and proteins function outside the host cell wall and improve the establishment efficiency of mycorrhizal symbiosis. Here we focus on the functions of these signaling molecules and secreted proteins released from mycorrhizal fungi at the early stage of mycorrhiza formation. Since global advances are much slower than those involved in pathogenic fungi, we hope the research in this field promotes deservedly.