Silicone rubber composite insulators are key equipment of HV transmission lines. Their surfaces will gradually deteriorate after running under a harsh environmental condition for a long time, and become chalked, faded, more rough and tough. As one of the characteristics of aging, surface roughness measuring is always difficult in power transmission line condition monitoring. Laser induced breakdown spectroscopy(LIBS) is suitable for remote condition monitoring in power system, but the influence of roughness on LIBS signal hasn't been systematically studied, and taking advantage of this matrix effect for insulator surface roughness testing hasn't been reported yet. New silicone rubber samples with different surface roughness were made, compared with those retired from a 500 kV power transmission line. The influence of roughness of new silicone rubber material on LIBS signal was studied and the results showed that when roughness increases, the spectral line intensity increases, while the ratio of different element line intensity(Si 288.2 nm/C 247 nm and Al 394 nm/Si 288.2 nm) decreases. It showed that roughness has a significant effect on LIBS signal. However, since there had been no obvious linear relationship between intensity and roughness or intensity ratio and roughness, it was hard to measure roughness by these relationships.The major elements of silicone rubber material were Si, Al, C, O and so on. Si was selected as the analytical element considering element content and spectral line selection convenience. Two atomic line of Si（SiⅠ288.2 nm, SiⅠ250.7 nm） was selected as the analytical line whose upper energy levels were nearly the same (Eki=40 991.88 and 39 955.05 cm-1). When the laser induced plasma satisfies LTE and optically thin condition, the ratio of line intensity should be constant, but the ablation process is affected by material surface roughness, and then the plasma state and intensity ratio will be changed. A calibration line of roughness and the above ratio was established and the linear correlation coefficient was 0.88. As for the aged silicone rubber material retired from the 500kV transmission line, the surface was precipitated by some ATH fillers because of the aging process, leading to a more inhomogeneous content in the matrix and a rougher surface. So it is not practical to measure roughness by only one couple of spectral lines. Therefor it is necessary to introduce Al atomic lines (Al Ⅰ 305.7 nm, Al Ⅰ 305.9 nm) in the calculation model besides Si atomic lines for the aged silicone rubber material. Multivariate regression was carried on with 3 sets of spectral intensity ratios, and then 2 agedsilicone rubber material were measured by LIBS whose actual roughness were 2.659 and 2.523 μm. The relative error was 0.218 and 0.189, respectively. The results show that roughness cannot be ignored in LIBS measurement for material with the same constitution, and the exploitation of this matrix effect for the remote measuring of composite insulators is meaningful for the checking and operation of HV transmission lines.