The invention provides an online detecting device and method for granules and liquid drops in gas pipelines. The online detecting method comprises the following steps: a sampling device collects a gas in a gas pipeline and introduces the collected gas into an optical device; granules and liquid drops contained in the gas penetrate through a sensitive area of the optical device to induce light scattering to produce scattering light signals; the optical device collects the scattering light signals in -90 degree direction and the scattering light signals in a certain corner or multiple corners in 0-180 degree directions, and divides the scattering light signals in a certain corner or multiple corners in 0-180 degree directions into P optical signals and S optical signals; the scattering light signals in the -90 degree direction, the P optical signals and the S optical signals are converted into electric signals in the -90 degree direction, P electric signals and S electric signals by a photoelectric conversion unit; a data processing device distinguishes the granules and the liquid drops in particles according to the P electric signals and the S electric signals, provides the shape properties of the particles and measures the particle size distribution and concentration of the granules and/or the liquid drops according to the electric signals in the -90 degree direction.