to those with S/N(${\hbox{H$\alpha$}}$) $>$ 5, S/N(${\hbox{H$\beta$}}$) $>$ 5, S/N(${\hbox{[O\,{\sc iii}]}}\lambda3727$) $>$ 5, S/N(${\hbox{[O\,{\sc iii}]}}\lambda\lambda4959,5007$) $>$ 5, S/N(${\hbox{[N\,{\sc ii}]}}\lambda 6583$) $>$ 3 and equivalent width (EW) of ${\hbox{H$\alpha$}}$ larger than 10 $\rm \AA$. Meanwhile, in order to obtain reliable stellar mass surface density, we select the spectra with a continuum S/N higher than 3 at 5500 $\mathrm{\AA}$. We exclude the spaxels affected by the active galactic nucleus (AGN) using the [@Kauffmann2003] demarcation line in the BPT diagram [@Baldwin1981; @Kewley2001; @Kauffmann2003]. We also exclude these galaxies, in which the selected spaxels number is less than 10$\%$ of the spaxels in this galaxy.

As a result, we achieve 1122 blue galaxies with about 750,000 useful spaxels. In Figure \[fig:bpt\_manga\], the $left$ panel shows total stellar mass – SFR (main sequence) relation for MaNGA galaxies, in which the $M_*$ and SFR are obtained from MPA/JHU catalogue [^3] [@Kauffmann2003; @Salim2007]. The blue points represent our final 1122 sample galaxies, while the grey points show all of the galaxies in MaNGA data. The main sequence relation for star-forming galaxies in local universe [@Salim2007; @Speagle2014] is shown as the black solid line. The $right$ panel shows the BPT diagram constructed with all spaxels in our sample 