Prosecutors say Mr Mieraili delivered the backpack containing the bomb to another man.
On arriving at court, he said he was "an innocent Muslim" who had already been detained for six months.
Both men arrived at court shackled and with bare feet.
Police chief Chakthip Chaijinda said the men had a right to retract their confession, but "we have witnesses and clear evidence to prosecute them", AFP reports.
The Thai military government says it suspects a people-smuggling gang angered by a crackdown on its activities carried out the attack.
But many observers believe the attack was in fact carried out in retaliation for the forced repatriation from Thailand last July of more than 100 Uighur asylum seekers back to China.
Fifteen suspects for whom the police issued arrest warrants are still at large.