Q:

Is it true that endianness only affects the memory layout of numbers,but not string? 

Is it true that whether the architecture is big or little endian ,only the memory layout of numbers differ,that of the string is the same. 

A:

If you have a simple 8-bit character representation (e.g. extended ASCII), then no, endianness does not affect the layout, because each character is one byte. 
If you have a multi-byte representation, such as UTF-16, then yes, endianness is still important (see e.g. http://en.wikipedia.org/wiki/UTF-16#Byte_order_encoding_schemes).