To investigate the improvement effect and mechanism of the combined of basalt fiber and fly ash on the tensile performance of cementitious soil, basalt fibers of various contents, lengths and fly ash of different contents were selected and incorporated into the cement soil consisting of weak soil in mountainous areas. A series of splitting tensile tests and electron microscopy scanning tests were conducted to study the strength, deformation, toughness, and microscopic mechanism of action. The results indicate that basalt fiber length 6 mm and doping amount 0.6%, and fly ash content 12% are the combined effect of the two has the best effect on improving the tensile performance and toughness of cement-soil. Compared with ordinary cement-soil, its tensile strength, residual strength, peak strain, and tensile absorption energy have increased by 40.55%, 134.68%, 40.81%, and 132.22%, respectively. The 18 mm fibers deteriorate the tensile properties of cementitious soil. The mechanism is that fibers reinforce the weak surface inside the cement soil, bear and transmit tensile stress, making the internal stress of the cement soil more uniform. Fly ash pearl particles can fill the pores inside the cement soil, making the cement soil particles fully bond with fibers, and enhancing the integrity of the sample.