The temperature dependence of VBL group formation in stripe domain walls of epitaxial garnet bubble films was investigated by means of the method of single-pulse bias field. A critical temperature T02 was found, which relats to the parameters of bubble films. When T >T02, hard bubbles no longer form. It was also found that, when T02, there are two different stages for VBL group formation. Their demarcation temperature is T01. The experimental curves in the first stage have been qualitatively interpreted. And the difference of soft domain segment movements, which lead ro their hardening, in the two stages has been revealed by means of the method of double exposure photography.