1.  phenomenon:

    Try to download firmware to flash by Flash Programmer in Nios II, add .sof and .elf file and start . It cannot work when next time powered on.

    .sof file can be downloaded by quartus programmer, but the target connection in nios cannot work with information as follows:

   No Nios II target connection paths were located. Check connections and that a Nios II .sof is downloaded.

2. theory or reason:

    not clear yet.

3. solution

   since that programmer in quartus works out well, download .jic file as well:

 after this step finished, everything is OK.