A laser-based multimode detector is introduced. To gain the optimal detection,the optical configuration is designed and optimized. Based on the theoretical analysis,mathematical deduction and Matlab simulation,the optimum numerical range of the parameters of the optical configuration are obtained. In the thermal lens detection mode,generally,smaller waist radius induces stronger the thermal lens effect,on the other hand,the waist radius of the probe beam,the distance between the waist and the sample and the distance between the waist and the detecting plane synthetically determine the intensity of the acquired signal. In the retroreflected detection mode,the focus length of the focusing lens has to be short enough,and the distance between the lens and the capillary,which has to be less,influences the detection a lot,while the lens has to be far enough both from the laser source and the detection plane.Finally,the thermal lens impacts the retroreflected interference unapparently. To eliminate the influence strictly,a proper little distance to shift the two detecting channels can be adopted.