RTOS/MSP430F5529: TI-RTOS/MSP430 : SPI master module receive data is complete, After call SPI_transfer() again to receive only the last byte of data!

Void spiB1_master_taskFxn(UArg arg0, UArg arg1)
{
    /* Create I2C for usage */
    SPI_Params_init(&spiB1_master_Params);
    spiB1_master_Params.transferMode          = SPI_MODE_BLOCKING;
    //spiB1_master_Params.transferCallbackFxn   = spiB1_transferCallbackFxn;
    spiB1_master_Params.mode                  = SPI_MASTER;
    spiB1_master_Params.dataSize              = 8;
    //spiB1_master_Params.frameFormat   = SPI_POL1_PHA0;

    spiB1_master = SPI_open(Board_SPIB1_MASTER, &spiB1_master_Params);
    if (spiB1_master == NULL) {
        System_abort("Error Initializing SPI B1\n");
    }
    else {
        System_printf("SPI B1 Initialized!\n");
    }
    System_flush();

    //gatemute
    GateMutex_Handle gatemutex2_handle;
    GateMutex_Params gatemutex2_params;

    GateMutex_Params_init(&gatemutex2_params);
    gatemutex2_handle = GateMutex_create(&gatemutex2_params, NULL);
    if (gatemutex2_handle == NULL) {
        System_abort("Error GateMutex2_create\n");
    }
    else {
        System_printf("GateMutex2_create OK!\n");
    }

    SPI_Transaction spiTransaction;
    IArg key;

    while(1){

        //
        key = GateMutex_enter(gatemutex2_handle);

        //cs low
        Board_SPIB1_MASTER_CS_LOW;

        char rxbuf[7];
        spiTransaction.txBuf = NULL;
        spiTransaction.rxBuf = rxbuf;
        spiTransaction.count = 6;

        if (SPI_transfer(spiB1_master, &spiTransaction) == NULL) {
            System_printf("SPI Bus fault\n");
            System_flush();
        }

        //cs hight
        Board_SPIB1_MASTER_CS_HIGH;

        rxbuf[6] = 0;
        printf("TASK spiB1_master receive DATA:%s\r\n",rxbuf);

        //
        GateMutex_leave(gatemutex2_handle, key);



        Task_sleep(1000);
        GPIO_toggle(Board_LED_P4_7);;
    }
}


int main(void)
{
    /* Call board init functions */
    Board_initGeneral();
    Board_initGPIO();
    Board_initUART();
    Board_initI2C();
    Board_initSPI();

    //spiB1 master
    taskParams.arg0         = 1000;
    taskParams.stackSize    = TASKSTACKSIZE;
    taskParams.stack        = &spiB1_master_taskStack;
    Task_construct(&spiB1_master_taskStruct, (Task_FuncPtr)spiB1_master_taskFxn, &taskParams, NULL);   

    /* Start BIOS */
    BIOS_start();

    return (0);
}