Background: Microsatellite instability (MSI) is a hypermutated phenotype primarily caused by the deficiency of DNA mismatch repair activity. Conventional MSI detection platforms are time consuming and need extra normal tissue. Due to the short DNA sequences of microsatellite loci in MSI samples, the method can be simplified to a one-step approach to differentiate MSI status by observing the melting curve. Herein a novel feasible MSI status detection method based on one step-PCR in single tumor tissue was developed, and validated in colorectal cancer samples. Methods: Specific microsatellite sites shorter than 12 bp in length (feasibility of fluorescence probes) were selected from public databases for analysis. In tumor tissues from 98 CRC patients with known MSI status, microsatellite sites with the highest receiver operating characteristic (ROC)-area under the ROC curve (AUC) were eventually selected for the final PCR panel design. The cut-off value of this assay was defined by the number of melting peaks in a specific Tm value range. An independent set of 199 tumor tissues from CRC patients with known MSI by Sanger sequencing was recruited to validate the performance of this novel PCR MSI kit. Results: The ROC-AUCs of the pre-selected eight mononucleotide repeats were 0.82, 0.93, 0.79, 0.78, 0.98, 0.95, 0.92, and 0.79. When 0 or 1 marker had a melting peak within the specified Tm value range, the sample was identified as "MSS/MSI-low (MSI-L)"; when 2 or more markers had melting peaks within the specified Tm value range, the sample was determined to be "MSI-high (MSI-H)". In the clinical sample set, compared with the results given by conventional MSI detection method, the sensitivity of the 8-gene panel for MSI-H was 99.4% (166/167), the specificity of the 8-gene panel was 100%, and the overall concordance reached to 99.5%. Conclusions: The novel PCR-MSI detection method using 8-gene panel developed in this study was feasible and accessible in clinical practice, and a large-scale clinical validation is in progress.