Thermal conductivity (kappa) is a key factor influencing the sensitivity of photothermal techniques. In thermal lens spectroscopy (TLS), photothermal signal was previously regarded as being proportional to 1/kappa. Here, I show that the 1/kappa dependence of photothermal signal is only valid on condition that the thermal diffusion length (L-D) exceeds quintuple the excitation beam radius (a(e)) for an ensemble sample or twice the probe beam radius (r(ps)) for a nanoparticle. When L-D is smaller than 0.5a(e) for ensemble sample (or r(ps)/6 for nanoparticle), the signal reaches a plateau (or starts to decrease), totally deviating from the 1/kappa dependence. It was experimentally demonstrated that at high modulation frequencies, the signal enhancement induced by organic solvent can be twice smaller than that predicted previously. This work illuminates the impact of kappa on TLS, and provides a guidance on selecting optimal TLS parameters for sensitive nonfluorescent target detection.