In order to explore the cascade effect of deuterium fluoride (DF) laser and the influence of working gas parameters on laser spectrum, the spectral characteristics of non-chain DF laser pumped by self-initiated volume discharge are studied. The pulse shapes of cascade lasing on P3(7) →P2(8) →P1(9) are measured by DF laser spectrum analyzer and HgCdTe photo detector, and it′ s found that the lasing line of P1(9) is initiated firstly, and P2(8) and P3(7) lines appeare 80~100 ns later. The number of laser lines and the relative energies are measured under different ratios and pressures of gas mixture, and 22 laser lines in the rang of 3.5~4.2 μm are attained. When the pressure ratio and total pressure of gas mixture (SF6 and D2) are 6:1 and 8.1 kPa, 25.9% of emitted energy is contained in the cascade lasing on P3(8) →P2(9) →P1(10). The results show that there exits cascade effect in multiline DF laser, and the extraction efficiency of laser gain with cascade output can be greatly increased by optimizing the parameters of gas mixture.