The conventional sound speed profile (SSP) inversion algorithms based on sound transmission time influence the inversion speed because they have to trace the eigen-ray of every SSP. The actual transmission time was expressed as the sum of that for referenced SSP and perturbation terms, and SSP inversion was changed from non-linear optimization to the solution of linear equations by the perturbation method. The experimental inversion results indicate that although the root mean square (RMS) error is doubled in comparison with that obtained by conventional algorithms, the calculating time can be reduced from 10 hours to 3 seconds by the perturbation method and therefore satisfies realtime monitoring of SSP.