By default it’s not possible to render ‘Bloom’ effect in the EEVEE with transparent background. But you can achieve it in a few simple steps.
Enable Bloom pass option in View Layer before rendering.
Under ‘Compositing’ tab be sure to enable ‘Use Nodes’ checkbox. Add a ‘ColorRamp’ node to control the ‘Bloom’ effect intensity. Connect the rendered layer alpha to the ‘Color’ node and switch its mode from ‘Mix’ to ‘Add’. With a ‘Set Alpha’ node replace the original image color pass with our new alpha pass.
💈 That’s it! The image is ready to be rendered. Also you can check youtube version here .