Zhuoyi,
This may help with your understanding of SPI and STE.
The STE pin is basically used as input both on SPI master and SPI slave mode: http://processors.wiki.ti.com/index.php/MSP430_FAQ#How_does_the_4_pin_SPI_work_on_USCI_module.3F
If you want to use the USCI module in SPI mode to communicate with external flash memory, which is basically it should act as SPI master, the best way is to use 3 pin mode and a GPIO for controlling the slave enable pin of the external flash memory.
Take care,
Jon
