Harmful　algal　blooms　have　become　a　global　environmental　problem.　Ultrasonic　technology　was　found　to　effectively　inhibit　the　algal　growth　and　reproduction　of　cyanobacteria　in　eutrophic　water.　Given　that　cyanobacterial　blooms　are　difficult　to　treat,　and　treatment　methods　may　cause　problems　leading　to　secondary　pollution,　a　preventive　control　was　proposed.　In　this　research,　algal　cell　sedimentation　was　performed　to　analyze　the　effects　of　ultrasonic　technology　on　algae.　The　photosynthetic　system　of　algae　and　the　optimal　combination　of　the　ultrasonic　removal　of　Microcystis　aeruginosa　were　also　studied.　Low-frequency　ultrasound　(20,　40　and　100　kHz)　was　selected　to　analyze　the　changes　in　phycobiliprotein　content,　the　expression　level　of　the　key　gene　in　photosynthesis,　and　the　settlement　index　after　treatment　with　different　ultrasonic　frequencies　and　power　densities.　Results　showed　that　the　phycobiliprotein　content　and　gene　expression　level　decreased　after　ultrasonic　treatment,　and　a　large　quantity　of　cells　settled.　The　inhibition　effectiveness　at　40　kHz　was　much　better　than　that　at　20　and　100　kHz.　When　the　power　density　of　0.02　W/mL　decreased　and　was　applied　at　a　frequency　of　40　kHz,　the　relative　content　of　phycobiliprotein　decreased　significantly,　and　the　phycobiliprotein　level　increased　by　44%-45%　compared　with　that　of　the　control　group　after　10　d.　Relative　quantifications　(RQs)　of　the　rbcL　gene　(encoding　RUBISCO　protein)　and　cpc　gene　(encoding　phycocyanin　protein)　were　inhibited　with　a　frequency　of　40　kHz.　The　RQ　value　decreased　by　more　than　40%.　When　the　power　density　remained　constant　and　was　applied　with　a　frequency　of　40　kHz,　both　the　algal　cell　sedimentation　index　and　the　inhibitory　effect　on　algal　growth　increased.　Thus,　ultrasound　with　frequency　of　40　kHz　and　intervals　of　4-6　d　is　suggested　when　dealing　with　algal　cells　to　prevent　the　algal　biomass　below　the　level　of　algal　blooms　from　achieving　preventive　restraint,　which　is　the　ability　of　algal　cells　to　self-repair　and　reproliferate.