Based on the principles of particle size measurement by multi-wavelength light extinction and image methods, a method for synchronous measurement of trans-micron scale mixed particle size is proposed, and the extinction spectral inversion algorithm for submicron-ten microns particle size and the image processing algorithm handling particle size over ten microns are established. The device for synchronously measuring extinction spectra and backlight images is established with a dispersion prism. The experiment with the trans-micron scale mixed particle samples prepared by ten kinds of standard particles with the sizes of 500 nm-76.9 μm is conducted. The results show that when the method is used to synchronously measure the trans-micron scale mixed particle size, the interaction between the extinction spectrum of submicron-ten microns scale particles and the backlight image of particles over ten microns can be ignored, and the trans-micron scale mixed particle size can be measured synchronously. The extinction method and the image method are respectively used to measure submicron-ten microns particle size and particle size over ten microns, the relative errors are less than 8% compared with those with the standard particle size, and the measurement repeatability is good, which provides an effective particle size measurement method for trans-micron scale mixed particles.