The invention relates to a biobased biodegradable fiber and a preparation method thereof. The invention aims at overcoming various defects in the existing biobased biodegradable macromolecule spinning technology. The biobased biodegradable fiber contains a PHA (Polyhydroxyalkanoate) homopolymer or copolymer with 5-75 percents of volume crystallinity and a composition of a PLA (Polylacticacidfiber) homopolymer or copolymer with 5-65 percents of volume crystallinity; the PHA homopolymer or copolymer occupies 5-55 percents of the fibers in weight; and the PLA homopolymer or copolymer occupies 45-95 percents of the fibers in weight. The specific preparation method comprises the following steps of: firstly, respectively vacuum-drying the PHA homopolymer or copolymer and the PLA homopolymer or copolymer; then, carrying out physical mixing according to the proportion and carrying out melt spinning; and finally, postprocessing. The biobased biodegradable fiber has better spinnability at a lower spinning temperature and a higher spinning speed and also has higher mechanical strength and continuous, stable and softer hand feeling, and with the adoption of the preparation method of the biobased biodegradable fiber, the production efficiency can be effectively improved, and the cost can be reduced.