convert step down to step up transformer?
is there any power loss?(compare to step up transformer,actually I see
  there is some thin and thick copper wire binding in the transformer.
  Will this affect the power loss?


Primary and secondary share the same core so, the main thing to consider is whether the core saturates more when driven backwards. Here's a sort of worked example: -

If the primary winding is 10 henries inductance, on 240V AC supply the current will be: -

\$\dfrac{240V}{2\pi f L}\$ = 76 mA. 

Let's say the transformer had 1000 turns on the primary so ampere turns are 76.

The 12V secondary has one twentieth of the primary turns i.e. 50 and its inductance will be \$20^2\$ times smaller at 25 mH. Now, if you applied 12V AC to the secondary you'd get a magnetization current of: -

\$\dfrac{12V}{2\pi f L}\$ = 1528 mA. 

Ampere turns are 50 x 1.528 = 76 (i.e. just the same)