The cucumber of solar greenhouse was selected to study the effect of biochar of corn straw on fungal abundance of rhizosphere soil and root growth of cucumber in fruiting period. The influence mechanism was found by the test. In the test, treatments were amounts of 20t/hm2, 40t/hm2 and 60t/hm2 of biochar which were respectively applied to the soil of solar greenhouse. The results showed that root number, root volume and root activity were improved in various degrees, the fungal abundances of Ascomycota, Zygomycota were increased significantly, and the fungal abundances of Basidiomycota, Chytridiomycota and Glomeromycota were promoted but the proportion of hybrid bacterium was reduced greatly, when the amount of biochar was 20～60t/hm2 in fruiting period of cucumber. The results also showed that the fungal abundances of Incertae sedis 27, Chaetomiaceae, Microascaceae, Lasiosphaeriaceae, Pseudeurotiaceae, Onygenaceae, Gymnoascaceae, Nectriaceae, Incertae sedis 3, Cephalothecaceae, Coniochaetaceae, Mortierellaceae, Glomeraceae, Bolbitiaceae and Spizellomycetaceae were improved by the application of biochar. Results showed that the effect of biochar on promoting root growth was more obvious for the fungi of Incertae sedis 27, Chaetomiaceae, Pseudeurotiaceae, Cephalothecaceae and Mortierellaceae, and these fungi promoted the root growth of cucumber by means of organizing in the root, promoting root to absorb nutrient, increasing organic matter and nutrient content, and promoting the decomposition of organic material. By comprehensive comparison, the effect of application rate of 20t/hm2 of biochar was better than those of other treatments, and the treatment of 20t/hm2 significantly improved the root number, root volume and root activity by 20.52%, 50.73% and 16.11%, respectively, compared with the test control in fruiting period of cucumber.