Background and Aims Colorectal cancer (CRC) can metastasize to distant organs, leading to poor prognosis, with liver and lung metastases being the most common. However, simultaneous diagnosed liver and lung metastases from colorectal cancer (SLLMCRC) are rarely reported compared to isolated lung or liver metastases. Therefore, this study was conducted to explore the prognostic factors for patients with SLLMCRC and to develop a prognostic prediction model to provide reference for the selection of treatment plans and evaluation of therapeutic efficacy.Methods Data of patients diagnosed with SLLMCRC from 2010 to 2019 were extracted from the SEER database. After applying inclusion and exclusion criteria, 800 eligible patients were selected. These were randomly divided into a modeling cohort (560 cases) and a validation cohort (240 cases) in a 7∶3 ratio. The Cox proportional hazards regression model was used to identify independent risk factors for overall survival (OS) of SLLMCRC patients, while the Fine-Gray competitive risk model was employed to identify independent risk factors for cancer-specific survival (CSS). Prognostic nomograms for predicting OS and CSS were constructed based on these independent risk factors. The reliability of the models was validated using the consistency index, ROC curve, and calibration curve.Results There were no statistically significant differences in baseline factors between the modeling and validation cohorts (all P>0.05). Age (50-69 years: HR=1.39, 95% CI=1.07-1.81; ≥70 years: HR=1.94, 95% CI=1.46-2.58), primary tumor resection (HR=0.67, 95% CI=0.48-0.95), CEA level (HR=1.39, 95% CI=1.04-1.87), and chemotherapy (HR=0.22, 95% CI=0.18-0.28) were independent factors affecting SLLMCRC patients OS (all P<0.05). The older age of SLLMCRC patients, the lower the OS rate, while having primary tumor resection, negative CEA result, and receiving chemotherapy result in higher OS rate. Age between 50-69 years (HR=1.05, 95% CI=1.01-1.12), number of regional lymph nodes removed (HR=0.67, 95% CI=0.48-0.90), and chemotherapy (HR=0.45, 95% CI=0.34-0.61) were independent factors affecting CSS (all P<0.05). Older age correlated with lower CSS rate, while more extensive regional lymph node removal and receiving chemotherapy correlated with higher CSS rate. The nomogram validation showed that the 1, 2, and 3-year OS ROC values for the modeling cohort were 0.643, 0.587, and 0.591, respectively; for the validation cohort, these values were 0.631, 0.623, and 0.628. The 1, 2, and 3-year CSS ROC values for the modeling cohort were 0.607, 0.610, and 0.681, respectively; for the validation cohort, these values were 0.624, 0.618, and 0.624. The calibration curves for OS and CSS were relatively close to the ideal 45° reference line.Conclusion Age, primary tumor resection, CEA level, number of regional lymph nodes removed, and chemotherapy are closely related to the prognosis of patients with SLLMCRC. Radiotherapy may not benefit SLLMCRC patients. The constructed prediction model has high accuracy and reliability, providing evidence to support clinical decision-making and evaluation of treatment efficacy for clinicians.