The invention relates to the technical field of internet bar mobile phone theft prevention, and discloses a desktop mobile phone anti-theft alarm device and alarm method for the internet bar. The device comprises a bottom plate, a placing box, four sliding rails, a placing mechanism, two elastic supporting mechanisms, an alarm triggering mechanism, two buzzers, a transparent cover plate, a double-control switch and an indicating lamp, the placing box is fixed at the top of the bottom plate; an opening is formed in the top of the placing box, every two of the four sliding rails form a group, the two groups of sliding rails are embedded in the left side and the right side of the placing box, the placing mechanism is slidably connected to the interior of the placing box, the two elastic supporting mechanisms are fixed to the two sides of the bottom of the placement mechanism, and the alarm triggering mechanism is fixed to the middle of the bottom of the placement mechanism. According to the invention, most of mobile phones of users can be placed conveniently, and when a stealer steals the mobile phone, the buzzer gives out alarm sound to remind the user to pay attention to whether themobile phone is safe or not so that the situation that the mobile phone is stolen is eradicated from the source.