GPS is widely used by civil aviation, financial institutions and transport sector and other civilian sectors, as well as for military purposes, thus the research on GPS spoof is of considerable merits. This paper analyzes the roots of forgeability and dupability of civil GPS signal; then, according to the characteristics of GPS civil signal, four forgery patterns of GPS spoofing signals are presented, which are navigation message forgery pattern, C/A code modulation forgery pattern, secondary carrier modulation forgery pattern and combination pattern; furthermore, two subpatterns of GPS navigation message forgery pattern, including GPS satellite signal fail forgery subpattern and GPS satellite fixing parameters forgery subpattern, are put forward with their detailed forgery techniques.