The effect of transverse stimulated Raman scattering (TSRS) in large-aperture KDP frequency-conversion crystals in high-power solid laser drivers operating at long pulse duration and higher energy fluence is studied. The relationship between maximum of the intensity and fluence of Stokes field produced during Raman scattering, and pulse widths of pump laser as well as crystal edge reflectivity have been presented. The result shows that the maximum intensity of scattered light always increases exponentially, regardless of pump laser duration and crystal edge reflectivity. The expression of the growth coefficient G has been presented. The maximum fluence of Stokes can be as high as 10 J/cm2 approaching the damage threshold of KDP crystals, once the growth coefficient G exceeds 25, where the risk evaluation rule of TSRS in large-aperture, high fluence KDP crystal is defined.