A single cylindrical lens configuration of the focus-error thermal lens technique is presented in this paper, and a theoretical model based on the optical phase shift theory is developed correspondingly to describe the focus error signal. The configuration optimization conditions obtained by the theoretical model show that the position of the cylindrical focusing lens coincides with that of the sample, which results in the difficulty of experiment. Therefore, the influence of the position of cylindrical focusing lens on the optimized values of other configuration parameters, focusing error signal and its nonlinear error are analyzed by numerical calculation. Numerical results show that when the distance between the focusing lens and the sample is less than 0.7 times of the focal length of the cylindrical focusing lens, the optimized values of the probe and excitation-beam radius are approximately unchanged, the distance between the detector and the focusing lens increases slightly, the focus-error signal is greater than 0.85 times, and the theoretical maximum value and the nonlinear error of the focus-error signal is less than 1% with a 0.2 rad of maximum optical phase shift of probe beam.