Objective: To find a more appropriate alternative to D-dimer cutoff value for the diagnosis of deep vein thrombosis (DVT) in cancer patients. Methods: A total of 711 cancer patients with symptoms suspicious of DVT were included in the study. D-dimer levels were assessed using ELISA. All patients were subjected to imaging procedures. Results: Among 711 patients with cancer, 466 (65.5%) were females and 245 (34.5%) were males, with an average age of $57.3{pm}13.23$ years. The mean age in the DVT group was significantly higher than in the non-DVT group (P<0.05). The D-dimer levels of the DVT group were significantly higher than those of the non-DVT group (P<0.05). The incidence rate of DVT varied significantly according to cancer type (P<0.05). Increasing age and lung cancer were significantly correlated with D-dimer levels (P<0.05), and a one-year increase in age was associated with a 14.28 ng/ml increase in the D-dimer value. The optimal cutoff point for D-dimer was found to be 981 ng/ml, with a sensitivity of 86.4%, specificity of 79.4%, and accuracy of 82.6%. If the D-dimer cutoff point was set to 981ng/ml, the specificity would increase from 61.8% to 85.5% without loss of sensitivity in patients aged 40 years or younger. In patients aged more than 40 years, the new cutoff almost doubled the specificity with slightly reduced sensitivity. Conclusion: In cancer patients, a new cutoff value of 981 ng/ml effectively improved the exclusion of DVT, especially for patients aged more than 40 years.