The development of biodegradable mulch film is an effective means to address plastic pollution and promote modern green agriculture. In this work, with compounding sodium carboxymethyl cellulose (CMC) and quaternized lignin (QL), a biodegradable liquid mulch film (PVA@CMC/QL) was constructed by introducing polyvinyl alcohol (PVA) and a selenium-containing cross-linking agent through electrostatic interaction. The effect of sodium carboxymethyl cellulose and QL on different liquid mulch films was examined. PVA@CMC/QL had exceptional spray-film-forming properties of liquid mulch film and was capable of generating a dense mulch film above the soil/on top of the soil under natural conditions. PVA@CMC/QL exhibited excellent oxygen transmission rate (60.2 cm·m·d·Pa) and water vapor transmission rate (753.4 g·m·d). Soil temperature and humidity increased by 0.4 to 2.1 °C and 0.5% to 2.8%, respectively, in the soil covered with PVA@CMC/QL compared to those in other controls, thereby confirming its exceptional moisture retention and insulation capabilities. PVA@CMC/QL combined remarkable weed suppression with only 13.3% weed germination under the mulch. Optimal rhizome growth of pak choi seedlings was observed under the PVA@CMC/QL cover, as demonstrated by the planting of both pak choi seedlings and weeds. Roots and stems increased by 3.8 ± 0.3 and 1.2 ± 0.3 cm, respectively. The weed suppression mechanism of PVA@CMC/QL was explained through the lens of density functional theory. In addition, the selenium content of pak choi seedlings under PVA@CMC/QL cover could reach 28.5 μg/kg, making the mulch film both degradable and highly reusable. This work not only improved the value-added utilization of bamboo residues but also gave new insight into the research on multifunctional bamboo-plastic mulch film.