This paper is concerned with a new concept of modeling the strain softening behavior of diatomaceous soft rocks based on the laboratory investigation. An approximate solution was described for a stress-strain-time relation taking into account strain softening phenomenon. The treatment of the post-peak behavior in the calculation is discussed and comparisons of the experimental observations of normally consolidated specimens with results of numerically simulated undrained triaxial tests are given. The strain rate effect and strain softening behaviors are controlled by the time dependent stress strain relation.