The invention discloses a diagnostic method and a diagnostic device for electrostatic 
waves of 
laser plasma instability. The diagnostic method includes the following steps: S1, generating an 
electron beam probe with an energy of more than 100 MeV and a 
beam length of 0.1 μm to 100 μm  ; S2, obtain the original density information of the 
electron beam probe; S3, excite the unstable electrostatic wave of the 
laser plasma, inject the 
electron beam probe with known original density information into the electrostatic wave, and make the electron beam probe pass through the 
plasma  Density modulation is generated under the action of the electrostatic wave 
electric field; S4, obtain the 
spatial distribution information of the density modulation of the electron beam probe after passing through the electrostatic wave; S5, invert and reconstruct the 
spatial distribution information of the density modulation, and obtain the absolute intensity information of the electrostatic wave  .  The invention solves the problems in the prior art that it is difficult to build a diagnosis platform, the absolute intensity information of electrostatic 
waves cannot be obtained, and the simultaneous diagnosis of two or more 
laser plasma unstable electrostatic 
waves occurring at the same position cannot be performed at the same time.