why usb blaster need be set a max3378 （Level conversion chip）

There's two different situations to consider;

1) A USB-Blaster cable

This type of USB-Blaster gets plugged into a target board via the 10-pin JTAG header. The VCC_TARGET pin on this header powers a dual-supply buffer inside the programmer so that the logic levels driven onto the JTAG pins are appropriate for the target board.

2) On-board USB-Blaster

When the USB-Blaster is included on a PCB, there is no need for a level translator, since the designer of the board will ensure that the USB-Blaster JTAG logic levels (typically from a MAX series CPLD) are appropriate for the JTAG device attached to it.

If you were to use an on-board USB-Blaster schematic as the reference design for building a USB-Blaster cable, then you would need to *add* the level translator to your design.

Cheers,

Dave