Objective To analyze the clinical value of magnetic resonance imaging (MRI) combined with detection of serum alkaline phosphatase (ALP) and lactic dehydrogenase (LDH) in early diagnosis and prognosis assessment of osteosarcoma. Methods A total of seventy patients with suspected early osteosarcoma who were admitted to the hospital from January 2012 to February 2013 were enrolled in the study. All patients underwent MRI scan and detection of serum ALP and LDH. With surgical pathological results as the golden standard, the diagnostic sensitivity, specificity and accuracy rates of MRI, serum ALP and LDH, and MRI combined with serum ALP and LDH were compared. Patients with osteosarcomas were divided into the high expression group and the normal group according to serum ALP and LDH levels, and the 5-year survival rate was compared between the two groups. Results The diagnostic sensitivity, specificity, accuracy and Kappa value of MRI were 83.87%, 75.00%, 82.86% and 0.41. Those of serum ALP and LDH were 79.03%, 87.50%, 80.00% and 0.40. Those of MRI combined with serum ALP and LDH were 95.16%, 87.50%, 94.29% and 0.75. The sensitivity and accuracy of MRI combined with serum ALP and LDH were significantly higher than those of MRI or serum ALP and LDH (P<0.05). The 5-year survival rate of the normal expression group was significantly higher than that of the high expression group (P<0.05). Conclusion MRI combined with serum ALP and LDH is with high accuracy in early diagnosis of osteosarcoma. Serum ALP and LDH levels can provide important evidence for evaluation of the prognosis.