String length constraining from stochastic gravitational waves background

String length is a fundamental parameter in string theory. A strategy on how to determine it through experiments is proposed. Our work focuses on the stochastic gravitational waves from string gas cosmology. With the help of the Lambert W function, we find the non-Hagedorn phase is ruled out by the B-mode polarization in the cosmic microwave background. The spectrum from the Hagedorn phase with a logarithmic term is found to be unique. We propose a strategy on how to constrain the string length through stochastic gravitational waves. Considering the sensitivities of the current and the upcoming detectors, the string length is found to be lower than 7 $sim$ orders of the Planck scale.