This study was conducted to evaluate the application value of a degradable liquid fiducial marker (LFM) in image-guided radiotherapy. In vitro experiment: using a solid fiducial marker (SFM) as a reference, the visibility, artifact, and optimal injection volume of an LFM under different cone beam CT tube voltage conditions were evaluated. In vivo experiment: using the SFM as a reference, the stability and degradation status of the LFM in nude mice were evaluated. Nude mice implanted with tumor cells were randomly divided into four groups: single fraction radiotherapy group (16 Gy/fraction) without LFM injection, single fraction radiotherapy group (16 Gy/fraction) with LFM injection, 2 fractions radiotherapy group (8 Gy/fraction) with LFM injection, and 4 fractions radiotherapy group (4 Gy/fraction) with LFM injection. The impact of LFM on tumor growth was evaluated based on the irradiation results. Compared with SFM, the LFM artifacts were significantly smaller (all p<0.05), and the visibility met the clinical differentiation requirements. The best imaging quality was achieved when the injection volume was 10 μL. The displacement of the LFM centroid relative to the spinal cord in the nude mice was significantly greater than that of the gold fiducial marker ((0.22 ± 0.03) mm vs. (0.17 ± 0.02) mm, p<0.05); however, it was always smaller than a pixel size. The results indicated good stability. The actual degradation rate of the LFM was highly consistent with the theoretical degradation rate. The LFM had a relatively smaller impact on tumor growth in the single fraction radiotherapy group but a greater impact in the fractional radiotherapy groups. LFMs have certain clinical applications and promotional value, and they are expected to replace SFMs in the future.