The invention provides a water-soluble degradable fiber and a preparing method thereof. The water-soluble degradable fiber is prepared from, by weight, 30-55 parts of polylactic acid, 10-30 parts of chitosan, 10-35 parts of nano-silica modified polyvinyl alcohol, 5-35 parts of a compatilizer, 0.2-3 parts of a lubricating agent and 0.2-3 parts of an antioxidant. The degradable fiber has good fusionspinnability at the low spinning temperature of 200 DEG C or so and the high spinning speed of 500 m/min or above, the fiber material has certain mechanical strength and continuously stable soft handfeeling, the solubility of the fiber in water is controllable, and the using requirement can be met. The water-soluble degradable fiber is particularly suitable for fiber fracturing fluid in the oiland gas production process, the problems of easy shakeout, backflow of a propping agent during after-pressure test quick discharge and the like are solved, the stability of the fracturing fluid adopting the fiber is high, and the water-soluble degradable fiber has a wide application prospect for site construction.